MIG Welding Car Bodywork: A Complete Beginner’s Guide

There is a familiar moment in nearly every restoration: a rusted-out rocker panel is cut out, a fresh sheet of steel is clamped into place, the torch is lit, and either the metal burns a hole straight through or the whole panel bows out like a dish. Both failures are avoidable, and both come down to the same thing – heat control and fit-up. MIG welding is forgiving enough that a determined beginner can put a clean, durable patch on a car in an afternoon, but only once the machine settings, gas, wire diameter and technique are matched to the thin steel that car bodies are actually made from.

MIG Welding Car Bodywork: Why This Skill Runs Every Restoration

Yes, MIG welding is a realistic way for a home enthusiast to do car bodywork, provided you understand what the process is good at and where its limits begin. You can put a strong, lasting patch on a rusted rocker panel, floor pan, fender, quarter panel or door skin with a machine that costs less than a decent weekend tool purchase, and you can do it in an afternoon. What you should not expect is a factory-grade finish on your first try, or to safely handle every joint on a modern vehicle.

One bit of terminology first. MIG is the trade name for a process whose official designation is GMAW: Gas Metal Arc Welding. The wire comes off a spool, feeds through the torch and forms the electrode the arc strikes. An inert gas, usually a mix of argon and carbon dioxide, blankets the molten puddle and keeps the air out. In one sentence: filler metal (the wire) is melted and added to base metal (the steel you are joining) to create the joint.

MIG dominates automotive work for three unglamorous reasons. It is fast, forgiving on thin material, and produces a clean bead that needs very little cleanup. TIG can make a prettier weld, but it is slower, needs two hands and more skill, and costs more. A beginner can put down a decent MIG weld in an afternoon and keep improving for years.

What You Can Realistically Repair

Auto body sheet metal typically runs around 18 to 24 gauge, roughly 0.8 to 1.2 mm thick, which is exactly where MIG is most challenging. Thin steel heats up fast, so it can melt through before you have moved the torch. It is also where MIG shines. Miller’s automotive welding guidance notes that MIG can handle automotive tasks from 24-gauge up to 1/4-inch thick, and Ron Covell’s sheet metal walkthrough covers the technique in depth.

For a first-time welder, the realistic list includes:

• Rocker panels and door skins
• Floor pans and trunk floors
• Fenders and quarter panels
• Patch panels fabricated at home
• Radiator supports, brackets and non-structural inner panels

Leave these to a professional: frame rails, suspension mounting points, crash-critical joins, and anything involving high-strength or boron steel. Galvanised and modern high-strength steels need different gas, wire and technique choices than mild steel, so a specialist usually handles them better. Old chromate primers and coatings add a safety dimension too, covered in the safety chapter.

The Core Tension: Heat Is Your Enemy

Everything in this guide orbits one fact: thin steel rewards precision and punishes excess heat. Too much heat burns a hole, too little and the puddle freezes before it fuses. The difference between a flat, solid panel and a buckled, cracked mess is usually not skill or equipment. It is control of heat input and fit-up.

Join type matters for the same reason. A butt joint keeps both panels at the same thickness, which makes the joint easier to straighten and avoids trapping moisture inside a lap joint. A lap joint is faster and stronger in some ways, but the overlap creates a crevice that rusts from the inside out.

Key Terms You Will See Throughout

• GMAW: the formal name for MIG welding
• Filler metal: the consumable wire added to the joint
• Base metal: the parent steel being joined
• HAZ (heat-affected zone): the area beside the weld where the steel changed without melting
• Butt joint: two panels meeting edge to edge
• Lap joint: one panel overlapping the other
• Tack: a small temporary weld to hold pieces in position
• Stitch weld: short welds laid in sequence with cooling time between
• Plug weld: a weld through a hole in one panel into the panel beneath, MIG’s version of a factory spot weld

What This Guide Covers

You will learn how the process works and why each setting matters, what machine, wire and gas to buy without overspending, how to set parameters for each gauge of steel, how to strip rust and fit a panel properly, how to stitch to control distortion, how to diagnose and fix common defects, how MIG compares to TIG and flux core, how to weld safely in a home garage, and how to finish the job so the repair does not rust out from behind in three years.

There is also a straightforward economic argument. A rocker panel or quarter panel runs from a modest supplier price to several hundred dollars, and a donor panel means finding a wrecked car with the exact part, which is increasingly hard to do. Fabricating your own patch from sheet steel costs a few pounds of material and a couple of hours. Done properly, a MIG repair can outlast the factory seam it replaced, provided you seal the backside properly, which most DIY work skips.

The rest of this guide is how to get there.

How MIG Welding Actually Works on Thin Auto Steel

Before you touch a dial, it helps to know what is happening at the torch tip.

MIG is formally GMAW (Gas Metal Arc Welding): an arc fusion process that melts filler metal to join the parts. MIG and flux core differ in where the shielding comes from. In MIG a continuous consumable wire feeds from a spool, replacing the stick electrode of stick welding. Automatic feed and a continuous arc make it faster, cleaner and far less slag-prone, which is why MIG dominates body shops and home garages. Miller’s automotive MIG primer covers the same fundamentals.

Following the wire from spool to puddle

• Drive rollers – wire leaves the spool, runs through a plastic liner and passes between two knurled rollers that push it forward at a controlled speed.
• Contact tip – the copper tip the wire slides out of. It carries current to the wire and slowly wears away, so it is a consumable like the nozzle.
• Gas nozzle – the larger cone-shaped shroud that directs shielding gas over the joint.

Polarity trips up newcomers. On solid automotive wire the torch is normally DCEP, electrode positive: the clamp goes to negative and the torch to positive, making the wire the negative side of the arc and the work the positive side. About two thirds of the arc heat lands on the work, exactly what you want when avoiding burn-through on thin panel steel.

With the arc struck, electrical energy melts the wire tip and the base metal surface, and the two molten pools merge into one puddle while shielding gas blankets it and keeps air away. Bare automotive steel is low-carbon and oxidises quickly, so air in the puddle means oxides, porosity, a weak bead and spatter everywhere nearby.

Short circuit versus spray: why body panels live in short circuit

In spray transfer the wire tip hovers over the puddle in a fine spray of droplets, giving a smooth, wide, deeply penetrated bead with very little spatter, but it needs relatively high amperage and higher voltage. On 20 gauge steel that mode is essentially unavailable to you, because the amperage needed to sustain a spray punches straight through the panel.

So thin automotive sheet runs on short circuit transfer. The wire feeds down until it touches the puddle, contact shorts the arc, resistance heats the tip and the wire melts, the pool freezes, the tip separates and the arc restrikes, hundreds of times per second. The result is a smaller, cooler bead with shallower penetration and a slight orange-peel texture, and it is the mode your machine settles into automatically at the low amperages you need for a rocker panel or floor patch. This guide to MIG on thin metal covers the same behaviour.

The four knobs and what they physically control

• Wire feed speed (WFS) sets how much filler metal arrives per second and therefore how much amperage the machine delivers. More wire means more heat and more deposition; feed too slowly and you get a cold, stacked bead the wire just sits on.
• Voltage sets arc length, and with it bead width and spatter. Usually set indirectly through the machine’s synergic mode, or on a separate control on a multi-process machine.
• Amperage drives penetration and heat input. Too little and the bead melts shallowly; too much and the panel disappears.
• Inductance is the machine’s resistance to sudden current changes. Higher inductance smooths the arc and suppresses spatter by limiting the current spike as the wire shorts, at the cost of a narrower, more deeply penetrated bead. On 0.6 mm wire, around 3 to 6 is the usual sweet spot.

The heat-affected zone

Heat does not stop at the weld centreline. The base metal on either side is heated, its grain structure changes and it is left hardened and stressed. That band is the heat-affected zone, or HAZ, and on thin low-carbon automotive steel that is where problems show up. Quench a panel with a damp rag, or leave it stressed and unsupported, and a hard HAZ plus tensile stress is where cracking appears, usually at the toe of the weld or the ends of the bead. Cracks often start in the HAZ rather than the weld metal, so let the panel cool naturally between passes.

Choosing the wire

Solid automotive wires are carbon steel classifications meeting the AWS A5.18 and ASME SFA 5.18 specs, so the naming is predictable: “E” means electrode, “R” a solid wire with a wire feed, “70” the tensile strength in ksi, “S” a single pass wire, and the last digit describes the deoxidiser and chemistry.

• ER70S-6 – the default. It carries more deoxidiser (manganese and silicon) than ER70S-3, which makes it the standard choice for hot-rolled, oxidised or lightly rusted automotive steel. Typical tensile strength runs about 85 to 90 ksi (586 to 621 MPa) in 75% argon / 25% CO2, with roughly 23 to 25% elongation. See the Lincoln Electric Pipeliner ER70S-6 data for the full spec sheet.
• ER70S-2 – higher deoxidiser levels again, for harder-to-weld steels and dirty or rusty joints.
• ER70S-3 – fewer deoxidisers and correspondingly more prone to porosity on contaminated steel. Still common in general fabrication.

And if your project is aluminium rather than steel, the whole approach changes: different feed equipment, different gas, different transfer behaviour. Aluminum MIG welding: the complete practical guide covers that side of the bench.

Picking Your Welder, Wire and Shielding Gas for Body Panels

Welding gear is where most home enthusiasts either overspend or buy something that fights them on every panel. Only three decisions really matter for car bodywork: how big the machine needs to be, which wire diameter you run, and what gas shields the arc. Get those right and an inexpensive welder beats an expensive one set up wrong.

120V or 220V: How Much Machine Do You Actually Need?

For sheet metal body repair, a machine in the 120 to 180 amp class is genuinely sufficient. Rocker panels, floor pans, fenders, quarter panels and door skins live in that range, and no amount of extra power improves a weld on 18 gauge steel. As Miller’s automotive MIG guide points out, thin automotive sheet has far more in common with the material used in ductwork and cabinets than with structural steel, and it rewards a soft, steady, low-heat arc rather than raw output.

A 120V single-input machine has real limits. It runs at half the input voltage of a 220V unit, so it pulls more current through the extension cord, gets hot faster and gives a less stable arc when pushed. That said, a 110V MIG is genuinely fine for body panels, and it is the right first purchase for a beginner in a residential garage where a 220V circuit may not exist.

Step up to 220V for steadier arc behaviour, fewer nuisance trips on a shared circuit, and headroom for frame and suspension work. The 240 to 300 amp machines in that class are overkill for sheet metal but sensible if the same machine will handle roll cages, chassis repair, exhaust work or trailer fabrication. Buying 300 amps to weld a rocker panel just gives you more heat available to get wrong.

Duty Cycle: The Number That Actually Matters

Duty cycle is simply how long the machine can weld before it overheats. A 20 percent rating means two minutes of welding out of every ten at full rated output; 40 percent means four out of ten. Twenty percent is workable for a quick patch panel. For a full-length rocker panel or long door skin you will be welding far more than two minutes straight, and a 40 percent or higher machine lets you work at your own pace without thermal protection cutting out halfway down the seam. Check the rating at the amperage you plan to use, not the headline number, because most machines quote a higher output at a lower duty cycle.

Multi-Process Machine or Dedicated MIG?

A multi-process unit gives you MIG, TIG and stick from one power source, and it is usually the better value if you want to expand your skills. The stick capability is no waste, because stick welding still earns its place for rusty seams, exhaust work and out-of-position repairs in a confined garage.

The trade-off is that TIG on a budget multi-process machine is often a simplified version, with limits on amperage control, no AC output for aluminium and sometimes no high-frequency start. A dedicated TIG machine costs more than a comparable MIG unit, but you get cleaner, more controllable welding with a proper foot pedal. For most readers starting out, buy the multi-process MIG now and add a separate TIG later if visible seams need that level of polish.

If you plan to touch aluminium panels, budget for a spool gun or at minimum an aluminium-rated liner and drive rolls, since aluminium work demands 100 percent argon. Our aluminum MIG welding guide covers the setup in more detail.

Wire Diameter: The Single Most Important Consumable Choice

Thinner wire means less heat and less burn-through, and on car bodies that is the entire game. Miller recommends 0.023 to 0.024 in wire for body panels and material up to 1/16 in thick, 0.030 in for 20 gauge through 1/8 in, and 0.035 in for 16 gauge through 1/4 in.

Wire diameter	Best suited to	Practical note
0.023 in (0.6 mm)	Body panels up to 1/16 in	The default for restoration work; lowest burn-through risk
0.030 in (0.8 mm)	20 gauge to 1/8 in	More versatile, slightly higher heat
0.035 in (0.9 mm)	16 gauge to 1/4 in	Heavier steel and structural work

Buy ER70S-6 solid wire in copper-coated form for steel bodywork. It suits dirty or lightly rusted automotive steel and meets the AWS A5.18 classification. Lincoln Electric’s ER70S-6 data covers a 1.0 mm (0.040 in) wire rated for 75 to 95 percent argon with the balance CO2, DC+, at 19 to 31 volts and 105 to 320 amps, which illustrates the point: a 140 to 180 amp machine sits comfortably inside that window, while the upper amperages only matter for thicker material.

Shielding Gas and Flow Rate

For steel bodywork, 75 percent argon and 25 percent CO2 (often sold as C25) is the standard blend. It gives a stable, forgiving arc that is easy to control on thin sheet, which is exactly what you want while learning. Pure CO2 produces a hotter, more penetrating arc and more spatter, which suits heavier structural work rather than visible panels. Pure argon is for aluminium only.

Set flow to roughly 20 to 25 CFH for MIG steel work, and add flow if you are somewhere drafty, because wind tears the gas blanket away from the puddle. Both MIG and TIG need a shielding gas supply, so budget for cylinders from your local supplier. Our guide to welding gas supply and cylinders covers sizes and handling.

The Running Costs Nobody Mentions

Shielding gas is usually the biggest ongoing expense, and the move up to a 20 lb cylinder is simple economics: you pay more for the container but less per cubic foot of shielding gas. Copper-coated ER70S-6 wire comes in 10 lb and 33 lb spools, with the larger spools costing less per pound. Then there are contact tips and nozzles, consumables that wear or melt, plus the wire liner and the ground clamp.

None of this is expensive in isolation, but it is the difference between a hobby that costs about the price of a good dinner every month and one that quietly eats your budget.

Machine Settings for Body Panels: A Table You Can Start From

Every MIG welding beginner hits the same wall: the machine spits out a number for wire feed speed, and that number means nothing until you know which panel you are holding. A rocker panel in 20 gauge sheet and a door skin in 18 gauge want different settings, and the gap between them is often only a few amps and a couple of volts.

The good news is that you are not guessing. Welding parameters for automotive sheet metal are well documented, and the table below gives you a defensible starting point you can adjust from in about ten minutes on your first panel. Treat these numbers as a starting position, not a target to hit perfectly. The real skill is reading the bead and correcting in real time.

Starting Settings by Material Thickness

Material Thickness	Wire Size	Shielding Gas	Amperage	Voltage	Wire Feed Speed	Inductance
24 gauge (0.5-0.6 mm)	0.023 in (0.6 mm)	75% Ar / 25% CO2	70-90 A	18-19 V	250-320 ipm	Low
20 gauge (0.8 mm)	0.023 in (0.6 mm)	75% Ar / 25% CO2	80-110 A	19-20 V	300-400 ipm	Low to Medium
20 gauge (0.8 mm)	0.030 in (0.8 mm)	75% Ar / 25% CO2	135-160 A	24-26 V	390-500 ipm	Medium
18 gauge (1.0 mm)	0.030 in (0.8 mm)	75% Ar / 25% CO2	150-190 A	25-27 V	450-580 ipm	Medium
16 gauge (1.2-1.6 mm)	0.030 in (0.8 mm)	75% Ar / 25% CO2	180-230 A	26-28 V	550-670 ipm	Medium
3 mm (0.118 in)	0.035 in (0.9 mm)	75% Ar / 25% CO2	200-250 A	26-28 V	600-750 ipm	Medium to High

These ranges are conservative for auto work because car panels deserve less heat than most shop settings assume. On a 220V machine with a wide voltage range, start at the lower end of the voltage range and let wire feed speed carry the amperage. Modern machines with a synergic or auto-set mode do this for you, and if yours has one, use it for your first few panels before dialling in manually.

What the Manufacturer Data Actually Says

For a sanity check against the wire manufacturer’s own numbers, Harris publishes a technical data sheet for ER70S-6 GMAW wire. ForER70S-6 wire in 0.023 in diameter, the rated operating range is 85 to 170 amps, 23 to 27 volts, 25 CFH of argon/CO2, and 360 to 620 inches per minute of wire feed speed. For 0.030 in wire, the range widens to 135 to 230 amps, 24 to 28 volts, and 390 to 670 ipm.

Notice something useful in there: the minimum wire feed speed on 0.030 in wire, 390 ipm, is close to where the 20 gauge row above starts. If your machine will not feed 0.030 in wire slowly enough to weld thin sheet, you need 0.023 in wire and a machine with good low-end feed control. Many budget 110V machines struggle here, which is why so many beginners get frustrated with wire feed on thin panels.

The Four Knobs and What Each One Physically Controls

You are really only adjusting four things on a MIG machine, and once you know what each one does to the puddle, settings stop being guesswork.

• Wire feed speed sets how much filler metal lands in the joint per second. Faster feed means more amperage, more heat, deeper penetration.
• Voltage controls arc length, and therefore bead width, how flat or crowned the bead is, and how much shielding you get. Too little voltage and the arc crowds the puddle and spits; too much and you get a wide, flat, rough bead with undercut.
• Amperage is the heat going into the panel. Too much and you burn through thin steel; too little and you get a cold, stacked-dime bead that sits on top of the metal.
• Inductance controls the rate at which current rises in the arc. Higher inductance smooths the transfer, widens the bead slightly, reduces spatter, and puts a bit more heat into the workpiece. Lower inductance gives a crisper, narrower bead with more spatter.

Reading the Bead and Correcting

Here is the diagnostic sequence most shops teach, and it works for home panels too.

• Cold, stacked-dime bead sitting on top of the metal: raise amperage or wire feed speed for more penetration.
• Wide, flat, rough bead with undercut and heavy spatter: raise voltage.
• Narrow, tall, crowned bead: lower voltage.
• Excessive spatter even though the settings look right: add inductance.
• Burn-through on 20 gauge: lower amperage and move the torch faster. Spreading the same heat over a longer distance is usually the fastest fix.

Gun Distance, Stickout and Travel Angle

Two distances matter and beginners constantly confuse them. The contact tip to work distance is the physical gap between the nozzle tip and the panel, and you want roughly 1/4 to 3/8 in. The electrical stickout is how far the wire sticks out past the contact tip, and for tacking you want about 1/2 in. Miller advises 1/4 to 3/8 in of wire stickout from the contact tip, and longer if access is tight on inner panels, roof rails and pillars.

A longer stickout acts as a small heat sink, so it cools the weld slightly and reduces burn-through risk. That is why experienced body shops can tack with a longer stickout and hold a very light touch that a beginner cannot reproduce with a short one.

Next is the angle. Hold the gun at 90 degrees to the workpiece with a 5 to 15 degree travel angle, which is the geometry Miller specifies for butt welds in automotive repair. Push or drag is a separate choice from that 90 degree position.

Push means you are dragging the nozzle forward with the arc pushing the puddle behind it. It gives lower penetration and a wider, flatter bead, better visibility of the joint ahead of the puddle, and less heat into the panel. Drag gives deeper penetration but more buildup, more heat and more spatter. Miller recommends push technique for thin sheet metal and body panels because it keeps the joint visible.

That is the whole method. Set the table, run a 3 in test coupon of the same gauge, judge the bead, and adjust one knob at a time. If the bead is not what you want, change only the voltage first, since it changes the arc character without changing how much metal you are adding. Then adjust wire feed speed for penetration. Save the combination that works on that gauge, write it on the side of your machine, and you will never have to think about it again.

Once the numbers are dialed in, the next challenge is not the weld. It is keeping the panel flat while you make it. That is where the stitch welding and sequencing technique comes in, and it makes a bigger difference to the finished repair than any setting on the machine.

Stripping Rust and Fitting the New Panel Properly

Welding is the loudest part of a body repair, but not what decides whether the job works. The deciding work happens before the torch is lit: finding rust, reaching clean metal, choosing the joint, and clamping the panel square.

Finding the Real Extent of the Rust

Rust is rarely confined to the hole you can see. It starts on lower edges where water collects: rocker panel bottoms, quarter panel bottoms, door bottoms, floor pans and wheelhouse arches. It also hides behind trim and under wheel arches.

• Probe every seam with a pick. Firm steel is solid; soft, crumbly metal is rust, often past the hole.
• Inspect pinch welds and flange edges. Rust starts at the factory seam and works outward.
• Remove trim and check behind it. Trim clips hide more corrosion than they show.
• Pe
  

  Stripping Rust and Fitting the New Panel Properly

  Welding is the loudest part of a body repair, but not what decides whether the job works. The deciding work happens before the torch is lit: finding rust, reaching clean metal, choosing the joint, and clamping the panel square.

  Finding the Real Extent of the Rust

  Rust is rarely confined to the hole you can see. It starts on lower edges where water collects: rocker panel bottoms, quarter panel bottoms, door bottoms, floor pans and wheelhouse arches. It also hides behind trim and under wheel arches.

  • Probe every seam with a pick. Firm steel is solid; soft, crumbly metal is rust, often past the hole.
  • Inspect pinch welds and flange edges. Rust starts at the factory seam and works outward.
  • Remove trim and check behind it. Trim clips hide more corrosion than they show.
  • Peel back wheel arch liners and shields, then check the inner wheelhouse and arch seam.
  • Tap with a hammer. A bright ring means solid steel; a dull thud means nothing behind.

  Most failed repairs start the same way: the panel was cut around the visible hole and the remaining rust was left in place.

  Getting Back to Bright Steel

  Paint, rust and oxide are electrical insulators and poor surfaces to strike an arc on. An arc landing on paint or rust wanders and spits, giving porosity instead of a fusion weld. Miller’s automotive MIG basics put it plainly: clean to bare metal first.

  Strip paint at least 1 to 2 inches on either side of the joint, and remove all rust plus any zinc or galvanised coating. A flap disc on an angle grinder clears surface rust and paint fast without thinning the sheet like a wire wheel. Zinc-coated material must come off entirely before hot work: it contaminates the weld and drives toxic zinc fume into your garage.

  Quick Note: A 60 to 80 grit flap disc strips surface rust and paint in seconds and leaves panel thickness alone. Save the 36 grit disc for the finished weld, and clean the ground clamp area in the same pass.

  Butt Joint or Lap Joint?

  For a visible panel, use a butt joint. A lap joint overlaps the new panel on the old one, and that overlap traps moisture inside it, so it rusts from within and the seam bubbles years later. It is also harder to straighten, since you planish two layers of steel. A roughly 3/8 inch lap joint suits hidden structure, subframe patches, or spots where you cannot hold a square butt. Eastwood’s sheet metal patch panel guide covers the trade-off in more detail.

  Making Your Own Patch Panel

  If there is no donor panel, making one is easy. Template the old panel in cardboard, transfer it to steel of matching gauge, then shrink and stretch the blank in a vice with a hammer to follow the curvature. Finish by planishing with a hammer and dolly until it matches the car. Ron Covell’s walkthrough of MIG and TIG welding auto body sheet metal shows the sequence.

  Gauge matters more than most people expect. Thin sheet needs proportionally less heat, so a patch thicker or thinner than the surrounding panel means chasing heat settings all afternoon and still risking a burn-through on one side or poor fusion on the other.

  Fit-Up: Where the Weld Is Won or Lost

  Clamp the panel up and check it before a single tack. Use clamps and Cleco fasteners, align it to the factory gaps and body lines, and compare it with the rest of the car.

  Fit-up check	Target
  Gap between edges	1/16 in (1.6 mm) max, or the weld will not bridge
  Tack spacing	1 in (25 mm) maximum apart
  Paint stripped at joint	1 to 2 in (25 to 50 mm) each side
  Ground clamp area	Bare, bright, unpainted metal
  Panel alignment	Checked against factory gaps

  Tacking no more than an inch apart is standard practice, and it is why a tacked panel comes out flat, not dished. Check alignment after every tack, correct it with a hammer and dolly, then weld. A curve across the panel now means over-tacking or overheating, easier to fix before the finishing weld.

  Grounding and Hemming

  Your ground clamp needs clean, unpainted metal. Paint and rust insulate, so the arc hunts and the clamp spits. Scrape a bare spot on a structural area, or clamp to the bare cut edge of the panel you are welding. Bad grounding is a top cause of erratic arc behaviour and spatter.

  On a replacement panel with a flange, hem the edge before fitting it. A hem folds the edge over and back on itself, so filler metal sits against a double thickness of steel instead of a feather edge that burns through. The pre-supported edge needs less heat, meaning less burn-through and less distortion, and it holds a consistent gap as the panel flexes.

  Note: Every metal wants its own prep. Steel wants bright metal with a wire brush or flap disc finish. Aluminum needs a dedicated chemical cleaner, as explained in Aluminum Cleaner for Welding: How to Prep Aluminum Right.

  And if the panel is still fighting you, consider the wire. Dropping from 0.8 mm to 0.6 mm ER70S-6 lowers the amperage you need and makes burn-through on body sheet far less likely, giving you room to fix a sloppy fit-up.

  Tack, Stitch and Sequence: Keeping Panels Flat

  Ask any fabricator what the hardest part of a body repair is, and it will not be the welding. It is keeping the panel flat while you weld it. Distortion is the tax on welding, but on automotive steel it is manageable: spread the heat around the joint instead of pouring it into one place.

  Why Welded Metal Moves

  The arc melts steel and that hot metal wants to expand, but the cold, stiff rest of the panel holds it back. When the weld and its heat-affected zone cool, they contract, and that contraction does the damage: the heated metal was stretched and is now pulling back, so the panel bows to let it. A long panel pulls along its length, and a door skin dishes where the heat sat in one place.

  Automotive welding guidance from Miller puts it plainly: welding almost always causes some distortion because metal in and around the weld shrinks as it cools. Distortion is not caused by a weak weld, nor cured by a thinner or faster bead. It is caused by how much heat goes into one spot at one time. Heat management is the cure.

  Quick Note: Three levers control distortion, and you will use all three: total heat input (amperage and travel speed), time at temperature (how long the metal stays glowing), and restraint (how tightly the sheet grips the hot zone). Loosening the panel’s grip on itself is often as effective as lowering amperage.

  Stitch Welding: Short Segments, Moving Around

  Instead of one continuous bead, weld short segments of about 20 mm (roughly an inch), jump elsewhere along the joint, weld another segment, then return to fill the gaps. Each small bead has less metal around it, so it expands and contracts without dragging the panel. Working in inch-long pieces around the joint is the standard approach for flat panels, and Thin metal MIG practice covers the principle in more detail.

  • Tack first. Fix the panel in position with small tacks before running any real seam.
  • Start in the middle of the longest seam, so movement is easier to absorb.
  • Alternate ends and corners. One short segment, then the opposite end or another corner.
  • Let it cool. Wait until the metal is no longer glowing before returning to it.
  • Fill the gaps last, once the seam is tack-stitched and the panel is flat.

  Sequence on Large Panels

  On a quarter panel or a full floor pan, order matters as much as segment length. Tack the panel at several points, check it against the car, then start the final seam. Work in 20 mm segments, alternating between opposite ends and between corners. Follow a natural curve or crown in a regular pattern, because regular spacing keeps shrinkage even.

  Between passes, give the panel time. A damp rag, cooling spray or a backing bar used as a heat sink speeds it up.

  Tacking and the Flush Test

  Tacks are temporary fasteners, not filler. Use quick, hot tacks with about a half inch of wire stickout: a short stickout runs hotter and sets the metal immediately, a longer one cools the weld and leaves it soft. After every tack, stop and look. Is the panel still where it belongs, and is the joint lined up? Correct anything that has moved with a hammer and dolly before you add another tack.

  Professional practice separates from home-shop impatience here: the joint is worked with a hammer and dolly until the metal is completely flush before the finishing weld goes down. Then check with a straightedge along the seam. A visible curve across the panel signals over-tacking or overheating, and it is far easier to fix while the panel is just tacked.

  For a flange that will not stay square, old body shops used the flagpole trick: tack a short length of scrap tube or angle iron to the panel edge so it holds the flange at the correct angle and spacing. Weld around it, knock it off, then dress the mark. The support carries the heat and holds the shape.

  Heat Sinking and Working From Below

  Where the joint is hard to reach, back the weld with a copper or aluminium bar clamped behind the seam, or clamp a thick bead of scrap metal against the underside. The bar absorbs heat and keeps the panel from getting hot enough to move.

  Where visibility allows, welding upside down is another useful habit, because heat rises and the panel distorts away from the side you can see. The seam finishes on the hidden face and the visible side stays flatter. The trade-off is working overhead with a puddle you cannot see directly.

  The Continuous Weld Alternative

  Sometimes a long bead is unavoidable, such as a pinch weld or a short structural seam. Then the answer is travel speed and inductance. Moving faster lays a thinner bead and spreads heat along the seam, while raising inductance softens the arc. Wire feed speeds for 0.030 in ER70S-6 wire are listed in the Lincoln Electric Pipeliner 70S-6 data, and most modern machines have an inductance or hold control.

  Short Note: Heat discipline matters even more on aluminum, which conducts heat far faster than steel. If you are asked to repair alloy wheels or aluminum trim, our Aluminum MIG welding guide covers why speed, stickout and chill bars become the whole game.

  Do Not Undo Your Own Work

  Grinding is a heat source too. A grinder left in one spot can bow a panel as badly as a hot weld. Keep the disc moving, work in short passes, let the panel cool, and dress welds in stages: coarse disc for bulk metal, finer discs to refine. If a panel that was flat before welding is not flat after, the cause was almost always heat applied too quickly in one place.

  I’ll rewrite this section, tightening it to fit the 600–900 word range while preserving the heading, key facts, and all links.

  Common MIG Bodywork Mistakes and How to Fix Them

  Most beginners treat a bad weld as a verdict rather than a diagnosis. The bead looks wrong, so they blame the machine, change three settings at once and end up with a second problem layered over the first. Trouble gets solved faster when you read the symptom like a symptom. A bead that pops off the plate, a hole in the panel, a line of pits along the seam: each one points to a specific cause, and each cause has a specific correction.

  Read the symptom, then hunt the cause

  What you see	Usual cause	What to change
  Burn-through, a hole in the panel	Too much heat for the gauge: amps high, travel too slow	Drop amps and wire feed, speed up travel, back the joint with a strip
  Bead sits on top, no fusion	Wire feed and amps too low	Raise wire feed speed first, then amps
  Wide, flat, rough bead	Voltage too high	Lower voltage 1 to 2 volts at a time
  Narrow, tall, crowned bead	Voltage too low	Raise voltage
  Groove cut along the toe	Travel too fast, arc too long, work angle off	Slow down, hold 90 degrees, keep the arc short
  Stacked-dime, cold bead	Travel speed too slow for the amperage	Increase wire feed speed and keep moving
  Spatter balls around the bead	Voltage high, dirty gas, wrong polarity, long stickout	Lower voltage, clean the gun, confirm DCEP, shorten stickout
  Pits, pinholes, wormy porosity	Contaminated gas, dirty base metal, leaking lines, long stickout	Check flowmeter, purge, clean the joint back to bright steel
  Cow hair, erratic feed	Worn drive rolls, tangled spool, snagged liner	Replace rolls, free the spool, blow out the liner
  Heavy orange spatter with flux core	FCAW is inherently spattery	Trim voltage slightly, use a drag angle, plan extra cleanup

  Burn-through and the melted hole

  Burn-through is the classic thin-metal failure and it is almost always a heat problem rather than a machine problem. The arc concentrates in one spot for too long, the puddle outgrows the metal underneath it and the panel gives way. Thin sheet burns far more readily than people expect because there is so little mass left behind to soak up the heat, a problem every fabricator working to thin metal has to manage.

  Four levers fix it, and they work in this order:

  1. Lower the amperage and wire feed speed. Running 90 amps on 20 gauge steel? Drop to 65 or 70 and see if the bead stops disappearing.
  2. Speed up your travel. Standing still is the single biggest cause of holes. Keep the torch moving at a steady pace.
  3. Add a backing strip. A piece of scrap steel or an old brake pedal behind the joint absorbs heat and stops the puddle punching through.
  4. Confirm the gas flow. Weak shielding lets the arc wander and concentrate. Check the flowmeter and hoses for leaks before touching a knob on the machine.

  Miller makes the same point about managing heat on body panels: a spot or spot-stitch weld while moving around the panel helps keep heat soaking and warping to a minimum, which beats trying to weld one long seam as cool as you can make it. A solid grounding of MIG automotive repair helps you understand why the same settings that ruin a floor pan will happily weld a frame rail.

  Spatter, cow hair and the wire that will not feed

  Spatter is reduced by three things, and you need all three: a shorter electrical stickout, correct DCEP polarity and stable gas flow. Push the wire out beyond roughly half an inch and the exposed tip oxidises, throwing molten balls across the panel. Not sure which polarity you are on? Run a short bead on a scrap piece. A clean, quiet arc with almost no spatter is DCEP. Run in reverse and you get a loud, spitting, deeply penetrated mess.

  Not all shielding gas is created equal, which is worth knowing before you blame the gun or the wire. Contaminated or improperly stored gas is a leading cause of both spatter and porosity, so it pays to understand what you are buying, and this guide to welding gas supply and cylinder choices covers the practical side.

  Cow hair, that wiry curl of wire trailing out of the gun, is a feeding problem rather than a welding problem. Check the drive rolls for grooves and flattened spots, make sure the spool is not tangled or spooling over itself, and blow out the liner. Thin wire is the usual culprit:0.030 in wire feeding through a 0.023 in gun will weave and spit.

  Undercut, cold beads and porosity

  Undercut is a groove melted into the edge of the joint, and on body panels it is usually caused by moving too fast or holding the work angle off. A pause at the side of a weave bead helps avoid it on thin panels, because you let the puddle fill the toe before dragging the arc along it.

  Porosity is the defect beginners find most alarming. Small pits and bubbles mean gas was trapped in the puddle. In MIG welds the cause is most often contaminated shielding gas, dirty or rusty base metal, or an excessive stickout. Clean the joint back to bright metal, purge with gas before you strike an arc, and shorten the stickout. The same rough logic applies if you work on aluminium, where the rules change considerably.

  A cold or stacked-dime bead means the amperage was right but you travelled too slowly, so the puddle froze before it spread. Raise wire feed speed, keep moving and the bead flattens out on its own.

  When the hole is already there

  Sooner or later the torch goes through. Two recovery routes work. The cleaner one is to cut the tear back to sound metal, slip a scrap piece of matching gauge over the hole, stitch it in place, then grind the scrap flush and treat it as part of the panel. The second is to grind the melted edge smooth and run a solid bead across the opening to close it, then lay your patch panel over that. Both are legitimate; patch-and-stitch usually produces the better result on visible panels.

  Ugly but sound: telling a bad joint from a bad-looking joint

  Plenty of first welds pass every structural test and still look terrible. If the bead is continuous, fully penetrated and free of cracks and pits, it is probably fine. Judge it by looking, not by feeling: a continuous bead of even width with slight reinforcement along the centreline is acceptable on almost any hidden panel.

  When the appearance genuinely bothers you, the answer is not filler. Body filler is a cosmetic skin over a problem, and it shrinks, cracks and falls off faster than a properly ground weld. Grinding a weld flat and doing it again properly is faster, stronger and better looking in the end.

  Quick Note: Fix one variable at a time. Change the voltage, make one test bead, look at it, then change the next thing. Welders who chase every knob at once take twice as long and learn nothing.

  One last diagnostic habit pays for itself quickly. Before blaming the machine, check the two things it is not: is the joint clean, and is the ground clamp on bare metal? A poor ground causes erratic arc behaviour, wandering bead and spatter that all look like a settings problem. Fix the ground and half the “machine faults” disappear.

  MIG vs TIG vs Flux Core: Which Process for Which Job

  Ask three experienced fabricators and you will get three confident answers, each delivered with a bit of tribalism: “MIG is for production, TIG is for real work, flux core is for barns.” For a car builder the picture is simpler. All three processes can make a sound repair. They trade speed against control, and control against convenience.

  MIG: the sensible default for home bodywork

  MIG (GMAW) is the fastest of the three, the cheapest per metre of weld and by far the easiest to learn, because the filler wire feeds automatically and you are only managing the torch. One machine covers 20 gauge body sheet right up to frame rails and suspension brackets. Its penalties are modest extra spatter compared with TIG and a heat-affected zone slightly wider than TIG would leave on the same panel. For most people learning to MIG weld car bodywork, that is a very good trade.

  Speed matters more on a car than people expect. Stitching a quarter panel seam in 20 mm segments with cooling time between each one adds up, and the extra minutes TIG costs you add up faster still. MIG also handles aluminium, but only on a machine rated for it with a spool gun fitted.

  TIG: the cleanest weld and the tightest heat control

  TIG (GTAW) produces the cleanest, most controllable bead and the most precise control of heat input of the three. Miller’s guide to MIG and TIG welding auto body sheet metal makes the point clearly: TIG’s real advantage on a car is heat control, which keeps the heat-affected zone narrow, minimises warping and leaves a bead you can hammer and dolly flush afterwards. It is also the only one of the three that handles stainless and aluminium cleanly without special technique. The price is slower travel, filler rod fed by hand with both hands, a hotter and more expensive machine, and pure argon.

  That is why Eastwood’s sheet metal patch panel guide states that both MIG and TIG are completely acceptable ways to make clean, reliable sheet metal repairs. The choice is about the job and your patience, not the quality of the finished repair.

  Flux core: the process for outdoors

  Flux core (FCAW) exists for one reason: it carries its own shielding agent inside the wire, so there is no gas cylinder and no wind problem. Wind blows shielding gas away and ruins a MIG or TIG weld outdoors, which is the single practical advantage that makes self-shielded flux core worth owning. On carbon steel it is fast, low cost and will happily weld dirty, rusted, mill-scale surfaces. The costs are slag and spatter requiring considerably more grinding and sanding than MIG, plus a noticeably rougher bead. Most experienced auto body practitioners advise against it on cosmetic panels. It also has specialised applications worth knowing, such as aluminum flux core welding.

  For completeness, stick (SMAW) shares the same outdoor advantages and adds its own; our look at the advantages of stick welding covers where it earns a place in a home shop.

  According to ESAB’s process comparison, MIG is fast, general purpose and moderate in cost, TIG is slow and precise with a higher equipment cost, and flux core is fast and low cost but limited in versatility.

  Process	Filler & gas	Speed	Bead cleanliness	Heat control	Cost	Best use on a car	Learning curve
  MIG (GMAW)	ER70S-6 solid wire, 75/25 Ar-CO2	Fastest	Clean, some spatter	Moderate	Lowest equipment and running cost	Nearly everything: patch panels, floor pans, rockers, frame repairs	Easiest
  TIG (GTAW)	ER70S-2 rod, pure argon	Slowest	Cleanest, virtually no spatter	Excellent	Highest equipment cost	Visible seams, stainless, aluminium, trim work	Steepest
  Flux core (FCAW)	E71T-11 self-shielded, no gas	Fast	Rough, slag and spatter	Moderate	Low per metre, no cylinder	Outdoor chassis and structural work, dirty steel	Moderate, cleanup is the work

  Resistance spot welding vs MIG plug welds

  Factories join body panels with resistance spot welds: two electrodes squeeze the sheets together and run a large current through them for a fraction of a second. You cannot reproduce that at home. A home plug weld uses a MIG plug weld instead, and there are two consequences. The plug is not sealed the way a factory weld is, so moisture reaches the joint and the bare steel around it rusts from the inside out. And a spot-welded factory seam sits inside factory seam sealer and cavity wax, while your plug weld sits on bare, freshly ground metal.

  Quick Note: After plug welding a structural or semi-structural joint, treat it exactly like any other MIG seam: epoxy prime, seam sealer or brushable seam wax over the weld, then cavity wax or drip-free undercoating into the surrounding cavity. That extra step is what stops the repair failing years later.

  What to learn, and in what order

  Start with MIG. It is the cheapest to learn, it covers the overwhelming majority of home car work, and skills like heat control, fit-up and reading a puddle transfer directly to everything else. Add TIG later if you want the cleanest possible seams on visible panels, or if you need to weld stainless or aluminium. Keep a flux core setup only if you genuinely weld outdoors on frame or structural work, where weather will eventually beat you.

  Whichever process you settle on, remember that the machine is the cheap part. Fit-up, surface preparation and corrosion protection are what determine whether the repair is still there in ten years.

  Welding Safety in a Home Garage: Fume, Gas and PPE

  When it comes to MIG welding car bodywork, the biggest threat is not the arc, it is what you cannot see. Fumes, gases and fire hazards build up fast in a home garage, especially when you are working with decades-old panels. A clean bead is worthless if it costs you your health, so safety belongs in your setup from day one.

  The Hidden Hazard: Fumes and Hexavalent Chromium

  Welding fume is a mixture of metal oxide particles, and its composition changes with both the base metal and any coating sitting on top of it. That matters far more in auto body work than in most other welding. Many older vehicles used primers containing chromate pigments, such as strontium chromate and zinc chromate, as anticorrosive agents. OSHA’s guidance on hexavalent chromium notes that hot work can convert chromium into its hexavalent state, Cr(VI).

  Because of its toxicity, OSHA regulates Cr(VI) exposure under 29 CFR 1910.1026, with a permissible exposure limit of 5 micrograms per cubic metre as an 8-hour time-weighted average. That is a tiny amount, so even short exposures to fumes from those primers can be hazardous.

  OSHA’s Welding, Cutting, and Brazing fact sheet specifically advises cleaning surfaces of any coating that could create toxic exposure, including solvent residue and paint. For body panels that means stripping old paint, chrome-containing coatings and chromate primers back to bright steel before striking an arc. It keeps chromium out of the hexavalent state and, as a bonus, gives you a far better weld.

  Ventilation: Control at the Source

  Fume control follows the hierarchy of controls, which puts engineering controls ahead of everything else. Source extraction is the gold standard: a fume extractor hood placed as close to the plume as possible, or a fume-extracting MIG gun for smaller, more frequent repairs.

  Some jobs make that much harder. Never weld in a confined space without ventilation, and treat welding inside a vehicle as a special case. Closed doors, floor pans blocking airflow and decades of coatings heating up can concentrate fumes at an alarming rate.

  Quick Note: CO2 shielding gas can displace oxygen and cause asphyxiation in confined or enclosed spaces. OSHA has noted that employers using carbon dioxide as a shielding gas, which is known to produce far higher amounts of welding fume, may not have met an engineering control requirement without adequate ventilation.

  Working from a home garage without a full extraction system? Massachusetts’ Safe Welding Practices recommends local exhaust ventilation as the primary control whenever possible. If extraction isn’t achievable for a given job, respiratory protection becomes critical.

  PPE: The Last Line of Defense

  Even with good ventilation, PPE is non-negotiable. The bare minimum for MIG welding body panels:

  • Welding helmet: an auto-darkening helmet with a shade range suited to the low amperages used on sheet metal.
  • Clothing: flame-resistant clothing, a leather apron, welding gloves, and safety glasses worn underneath the helmet.
  • Respiratory protection: when welding over existing coatings, the Massachusetts guide recommends a P95 particle respirator or better as a minimum, with organic vapour cartridges added for solvents and paint fumes. For frequent bodywork in those conditions, a P100 with organic vapour cartridges is the safer option.

  Most importantly, OSHA requires a written respiratory protection program with fit testing whenever respirators are required. A respirator only works if it seals properly to your face, so that step cannot be skipped.

  Gas Cylinders, Fire and Electrical Safety

  Pressurised cylinders and hot work bring their own risks. Keep these in mind:

  • Cylinder safety: secure the shielding gas cylinder upright with a chain or approved stand. Never lift a cylinder by the regulator, store or transport it badly, check the regulator and hose for leaks, and close the valve when not in use.
  • Fire safety: MIG itself is not a major fire risk, but the grinder and the arc are. Keep a Class A/B/C fire extinguisher within arm’s reach, use a welding blanket, and move rags, spray cans and other flammables clear of the work area.
  • Electrical and prep safety: ground the welder properly, check the cables for damage, and never weld in wet conditions. When you sand old paint to prep a panel, ventilate that dust too. Paint fume is just as hazardous before the arc is struck.

  Safety is not slowing your restoration down. It is the reason you are still welding years from now.

  From Weld to Show Car: Finishing, Priming and Rust Prevention

  A weld is only half of the repair. Plenty of home jobs produce a bead that would pass a press test and then fail anyway, two or three winters later, because the back of the joint was left as bare, cold-rolled steel. The metal you added is fine. What was missing is everything the factory did to that panel after it left the assembly line: grinding, filler, epoxy primer, seam sealer, cavity wax.

  So treat finishing as its own stage of MIG welding car bodywork, not as cleanup. Here is the order that works.

  Dressing the Weld: Coarse to Fine

  Start with bulk removal. A 3-inch 36-grit flap disc on an angle grinder takes down a proud weld bead fast. Follow with a 50-grit disc to smooth the surface, then blend it in with a 120-grit pad on an orbital sander. The orbital is the tool that really matters for panel gaps, because it lays a feathered, level pass instead of a visible grind line. Eastwood’s guide to sheet metal patch panel work walks through the same sequence.

  Quick Note: Keep the grinder moving. Grinding generates heat, and heat is what bows a panel. Work in short passes, let the disc cool, and never lean on the panel to “feel” the flat. If the sheet starts to flex, stop and let it cool completely.

  Leave a whisper of metal proud rather than grinding dead flush with the surrounding sheet. That slight crown gives filler something to bond to and leaves you a seam to sand without cutting through into the base metal.

  Body Filler: A Finishing Tool, Not a Structural One

  Polyester body filler is for raising low spots and blending the ground weld into the panel, nothing more. Mixed thin, spread over the seam and low areas, allowed to cure, then sanded flush with 80 to 120 grit. It shrinks slightly as it cures, so a second skim coat is usually better than one heavy coat.

  Be honest about its limits. Filler is not as strong as metal, it is brittle compared to steel, and it shrinks. Never use it to bridge a gap, replace a missing flange, or fill a structural seam such as a rocker or floor pan joint. If a load path needs metal, it gets metal. For pinholes and tiny voids, use a thin finishing glaze rather than a thick filler build-up.

  The Corrosion Layer Most DIY Repairs Skip

  This is the step that decides whether the repair lasts a decade or one wet winter. Work the inside of the joint first, not the outside.

  1. Clean the reverse of the joint. Take it back to bright steel and wipe it with a wax and grease remover. Sealant over dirty or oily steel just traps the moisture.
  2. Epoxy prime all bare steel. That includes the back of the joint, the panel edges and any ground areas where the original coating was removed. Two-part epoxy primer is the version that stands up to road salt and fuel.
  3. Seam sealer or brushable seam wax over the weld. Seam sealer matches the factory texture; brushable seam wax is easier in a home garage and creeps into crevices.
  4. Cavity wax or undercoating into the panel cavities. Spray it into the hollow sections of doors, quarters and fenders so condensation has no bare metal to attack.
  5. Drip-free underbody coating on floor pans and rockers. Choose drip-free over traditional tar, which eventually weeps onto the road and down the side of the car.

  Before any of that, the existing factory seam sealer, tar or e-coat has to come off the joint before the arc strikes. Welding through old sealant traps oil, silicone and moisture, and you get a porous, gas-filled weld that pits exactly where it is hardest to see. OSHA’s welding safety fact sheet also stresses that surfaces should be cleaned of any coating that could create toxic exposure once heated.

  Why a Raw Weld Rusts Sooner Than the Panel Around It

  The weld bead and its heat-affected zone do not behave exactly like the surrounding factory sheet. The HAZ has a different microstructure and residual stress, and on low-carbon automotive steel it can be slightly more prone to pitting at the toe of the weld. The bigger issue is simpler: a factory seam received sealer and cavity wax. A raw weld does not, unless you put them there. Get that wrong and the repair becomes the weak point on an otherwise sound car.

  That is also why paint alone is not a corrosion barrier. Epoxy primer is the barrier; the colour coat is appearance. Skip the primer and you are watching the topcoat blister within a season or two.

  Paint Order and the Final Check

  Epoxy primer over bare steel, then seam sealer, then colour and clear. Seal it, sand it lightly if the texture needs to match, and do not paint over loose dust or wax.

  Before you spray anything, walk around the car with a hammer and dolly and look for movement in the repaired area. Then check panel gaps against the factory reference on the opposite side of the car, and inspect the ends of every weld for hairline cracks. If the car is aluminium rather than steel anywhere in the repair, prep and protection change completely, as covered in this guide to prepping aluminium for welding.

  Then come back in two weeks, and again after a month, ideally after the car has sat through rain and a couple of temperature swings. Look for rust bloom at the seam edges, bubbling paint, or a gap that has opened up.

  Note: Miller’s automotive repair guidance makes the point that a properly completed repair, with filler, primer and paint, should last for decades. That outcome depends entirely on the steps most people skip. Weld correctly, then seal correctly, and the repair can outlast the seam it replaced.

  Frequently Asked Questions

  Is MIG welding suitable for car bodywork, or do I need TIG?

  MIG is the standard choice for most home body repairs. It is fast, forgiving and handles thin sheet well once you have the heat under control. TIG produces a cleaner, more precise bead and is worth learning later for visible panels, aluminium or stainless work, but it is slower and requires a more expensive machine.

  What is the best MIG setting for car body panels?

  Start with 0.023 in (0.6 mm) ER70S-6 wire, 75% argon / 25% CO2 gas at 20-25 CFH, and roughly 85-130 amps depending on gauge, with 23-27 volts and a wire feed speed around 360-620 ipm. Those are the manufacturer’s published starting ranges for 0.023 in wire, not a fixed answer – panel gauge, joint design and travel speed all shift the numbers.

  How do I stop burning holes in thin car panels?

  Lower the amperage, switch to thinner wire, shorten your stickout and move the gun faster along the seam. If you still blow through, weld the joint over a piece of scrap metal as a backing plate or clamp a bead of aluminium behind it to soak up the heat.

  Why does my panel warp after welding even though the weld looks fine?

  Distortion comes from uneven heating and constrained shrinkage, not from a bad weld. Stitch weld in short 20 mm segments, jump around the joint, and let the metal cool between passes. Tacking too close together in one spot is the other common cause of a buckled panel.

  Can I use flux core wire for body panels instead of buying shielding gas?

  It is possible but most experienced body workers advise against it on cosmetic panels. Flux core lays slag, throws more spatter and needs considerably more grinding, and it is harder to control heat on thin sheet. Its real advantage is outdoor structural work where wind would blow away the shielding gas.

  Do I need to worry about welding fumes when working on an old car?

  Yes. Old paint, primers and chrome-containing coatings can produce hexavalent chromium in the fume, and OSHA sets a permissible exposure limit of 5 micrograms per cubic metre for Cr(VI). Strip coatings back well beyond the weld area, use local exhaust ventilation close to the arc, and wear a properly rated respirator.

  Should I use a lap joint or a butt joint for a replacement panel?

  Butt joints are preferred on visible body panels because a lap traps moisture that can start hidden rust, and it is far easier to get flush and straighten. A small overlap is acceptable on hidden structure or where access makes welding a butt joint impossible.

  What gauge of steel is typical on car body panels, and can my welder handle it?

  Body sheet typically runs 18-24 gauge, about 0.5-1.2 mm. A 120-180 amp MIG welder handles that range comfortably with 0.023 or 0.030 in wire; you will only need a larger machine for frame rails, suspension mounts and thicker structural steel.

  Do I need to weld galvanised panels with something different?

  Galvanised steel needs a compatible wire and gas – a copper-free wire such as ER70S-6NC with a higher argon blend is often recommended because standard copper-coated wire can cause zinc contamination and porosity. Modern high-strength and boron steels need specialist low-hydrogen procedures, so those repairs are best left to a shop with the right equipment.

  What finishing steps stop a welded repair from rusting out again?

  Back the seam with epoxy primer, run seam sealer or brushable seam wax over the joint, and spray cavity wax or undercoating inside the panel cavities and along the rockers and floor pans. Paint alone is not a corrosion barrier, and a raw weld without cavity protection is the most common reason a good weld still fails years later.