Good MIG Welding: The Practical Guide to Clean, Strong Welds

You finish a bracket, hit the grinder, and a bright orange shower of sparks tells you the story is not over. The weld looked fine while it was cooling, but underneath it was porous, undercut, and barely fused to the base metal. That moment — when a MIG weld looks good and still isn’t — is the gap most welders never learn to close. Good MIG welding isn’t about owning the right machine; it’s about understanding what the arc, the gas, and the wire are doing while you’re moving them. This guide walks through that logic step by step, so your next weld looks right, tests right, and holds.

I’ll rewrite the section, keeping the heading, all facts, and every link, while tightening the prose into the 560–840 word range.

What Makes a MIG Weld “Good”? Setting the Standard Before You Strike an Arc

A good MIG weld passes two tests at once. It has sound metallurgical properties: full fusion to the base metal, adequate penetration, and no cracks or porosity hiding in the puddle. It also shows a clean, uniform surface profile a trained eye can verify in a second. Miss either half and you have an ugly weld that works or a beautiful weld that does not.

Shop floor talk uses “good” loosely. It helps to split it into the three things people actually mean.

Sense of “good”	What it really means	How it gets judged
Cosmetic	The bead looks clean: even ripples, consistent width, tidy toes, minimal spatter	Visual inspection by a person who knows what to look for
Mechanical	The joint carries real load: strength, ductility, and fatigue life match or exceed the base metal	Macro examination, tension and bend tests, and nondestructive testing
Code compliance	Made to a written procedure and meeting the acceptance criteria in the governing specification	Inspection against the WPS and the applicable AWS code

That third sense is where the paperwork lives. Every code job runs against a written procedure, recorded on a Welding Procedure Specification: the promise the shop makes in advance about settings, consumables, and technique, with inspection as the audit of that promise.

Why Appearance Is Only a Proxy

The uncomfortable truth about MIG: you judge a weld from the outside when the interesting failures are on the inside. A bead can be mirror-straight and perfectly rippled and still be only partially fused to the base metal at the root. So inspectors cut a cross section, macroetch it, and look at what happened inside. The FHWA Bridge Welding Reference Manual lists what a fillet weld macroetch must show: proper size, complete penetration to the root, complete fusion, no or minimal undercut, and an acceptable profile. Appearance is the first filter, not the last.

The Visual Vocabulary

To read a weld you need words for what you see. These are AWS terms, and using them precisely separates “it looks fine” from an actual inspection.

• Undercut a groove melted into the base metal at the toe of the weld where the bead fails to blend in.
• Porosity AWS defines it as “cavity-type discontinuities formed by gas entrapment during solidification or in a thermal spray deposit.” Bubbles that froze into the weld.
• Lack of fusion the weld metal did not melt and merge with the base metal or with the previous bead.
• Convexity the crown or reinforcement above the surrounding surface. AWS D1.5 limits convexity to 0.07 times the actual face width of the weld plus 1.5 mm (0.06 in.).
• Overlap a cold lap where the weld metal flows over the base metal without fusing to it.
• Cracks and, in AWS D1.1, cracks are unacceptable in welds regardless of size or location.

Undercut is not automatically a reject, but the limits are tight. Under AWS D1.5, undercut is limited to 0.25 mm (0.01 in.) when a weld is transverse to tensile stress, and 1 mm (1/32 in.) for all other cases.

Set expectations honestly. A weld can look beautiful and still fail, and a plainer-looking weld can be structurally superior.

The Machine Is Not the Variable That Matters Most

Newcomers assume weld quality is bought. It mostly is not. The strongest influence on a MIG weld is the technique and parameter discipline behind the gun, not the amperage range or the brand badge on the box. MIG is approachable because it exposes its four main variables in plain sight: wire feed speed, voltage, travel speed, and gas flow, each of which responds to your hand immediately and predictably. AWS calls the process GMAW, and a constant-voltage machine turns those four knobs into a simple mental model rather than guesswork.

The same standards apply to other metals with different consumables. If you are working aluminum, read our Aluminum MIG Welding: The Complete Practical Guide for gas and wire specifics, then come back here. The judging criteria never change.

Where This Guide Goes Next

We start with the mechanics of GMAW so the arc stops feeling mysterious, then wire and shielding gas, where most “good MIG welding” problems begin. From there: machine settings, then the hands, gun angle, work angle, travel control, and the leading edge principle. Then joint preparation and cleanliness, defect reading, a symptom-to-cause troubleshooting checklist, and the safety and ventilation work that keeps you welding for a career. We close with a practice plan and a buying checklist.

How MIG Welding Actually Works: The Process Behind the Picture

Before you can fix a weld, know what happens when the trigger is pulled. MIG welding earns its reputation by doing three things at once: melting a continuously fed wire electrode, joining it to the work with an arc, and blanketing the hot metal in shielding gas so the air cannot reach it. That is why Gas Metal Arc Welding (GMAW), as AWS calls it, allows faster travel speeds and higher deposition rates than stick welding.

MIG or MAG?

In strict American Welding Society terminology, GMAW covers the whole family. “MIG” means the inert-gas variant, with argon or a helium-argon mix as the shielding gas. “MAG” means the active-gas variant, where the shielding gas contains a reactive gas such as CO₂ that actively participates in the weld.

In practice the trade has called nearly everything MIG for decades. “Our MIG department” usually means GMAW on steel with a tri-mix or C25 blend. For aluminum or stainless the mechanics differ enough that a dedicated aluminum MIG welding guide is worth reading first.

The wire path, and where it fails

The most useful troubleshooting model: the wire is a consumable circuit. Follow it end to end and you find faults with your eyes.

1. The spool holds the wire. Solid MIG wire is usually copper-plated to prevent oxidation, aid conductivity, and extend contact tip life.
2. The drive rollers push the wire at a set speed. They wear, gum with lubricant, or sit too loose or too tight.
3. The liner guides the wire from feeder to gun, and it is the most under-ignored part of the system.
4. The contact tip is a consumable nozzle piece the wire passes through, and it transfers current to the wire.
5. The gun delivers wire and shielding gas to the work.
6. The work completes the circuit.

Every item on that list can quietly destroy a weld. A dented liner changes friction, so the wire feeds slower than the dial says and the current drops with it. A worn contact tip arcs to itself, throwing spatter onto the bead. Lubricant in the feeder or liner, a common cause of excessive spatter, contaminates the wire before it reaches the pool, and so does oil from careless handling.

Polarity: the switch that changes everything

Polarity decides which end of the DC supply is the electrode and which is the work. Solid wire with external shielding gas runs DCEP (DC+), wire positive and work negative. Self-shielded flux-cored wire such as E71T-GS or E71T-11 runs DCEN (DC−), wire negative and work positive. Get it backwards and the electrode melts faster than the base metal, the arc destabilises, and the deposit is weak and messy. Check the machine label and the spool before every setup.

Transfer modes: how metal crosses the gap

How molten metal crosses the gap is the transfer mode, and GMAW uses several:

• Short circuit, the default for thin steel and short welds. The wire tip touches the pool, current spikes, and the droplet transfers mechanically. Easy, cool, and low on spatter.
• Globular, where larger droplets fall off the wire in a less controlled way. Useful in heavier applications, messy in general.
• Spray transfer, where fine droplets are blown off the wire by arc force and land in a quiet, well-shaped bead. This is the high-deposition mode for flat, horizontal, and vertical-up work on thicker material, running on higher current and a costlier argon blend.
• Pulsed spray, which pulses current so the transfer looks like spray at lower settings, giving a spray-like bead while staying within short-circuit-friendly wire diameters and heat levels.

The one model that makes it click

Here is the rule to memorise: on a constant-voltage MIG machine, wire feed speed controls amperage and voltage controls arc length. Push the wire faster and you ask for more current, more heat, deeper penetration, and faster deposition. Raise the voltage and the arc stretches longer, widening the bead, flattening the profile, and improving edge wetting. Miller’s parameter guidance builds its tables on exactly this relationship.

Synergic machines automate it. You set wire feed speed, material thickness, and wire and gas type, and the machine programs the voltage for you. It helps only if you understand what it is doing, since you still adjust for position, joint design, and travel speed.

One ground rule ties the process together: the environment has to cooperate. AWS D1.1 sets ambient environment conditions for welding and does not permit welding when surfaces are wet or exposed to rain, snow, dust, or high wind. The shielding gas protects the pool from air, not from a storm.

The “70” in ER70S-6 denotes a minimum as-welded tensile strength of 70,000 psi, which is why that classification covers most structural and general fabrication work under AWS D1.1 and D1.3.

Choosing the Right Wire and Shielding Gas (and Why the Combo Matters)

Most people blame the machine when a weld goes wrong. But the two things you feed into the gun, the wire and the shielding gas, decide more about your bead than any setting you will dial in. Get the pair right and the job gets forgiving. Get it wrong and you spend all day fighting spatter, porosity and erratic penetration on a machine that works perfectly.

Reading the Wire Label: AWS A5.18 in 30 Seconds

Solid wire carries a classification such as ER70S-6. Every character tells you something useful, and once you can read it you can buy the right spool without guessing.

• E means electrode, R means rod or wire, so ER simply identifies a solid filler wire.
• 70 is the minimum tensile strength in thousands of psi, so ER70S-6 deposits metal that must pull at least 70,000 psi. It also signals a mild steel alloy.
• S means the wire is solid, as opposed to a tubular flux-cored wire.
• 6 is the deoxidizer chemistry level, and it decides how dirty the steel can be before the wire starts giving you trouble.

That last digit matters most in a shop environment. Deoxidizers such as manganese and silicon tie up oxygen and nitrogen in the weld pool so they cannot come back as porosity.

The Three Carbon Steel Wires You Will Actually Meet

Lincoln Electric publishes the chemistry for its ER70S-6, and those numbers explain why it has become the industry default. Specified manganese runs 1.40 to 1.85 percent and silicon 0.80 to 1.15 percent, with copper at 0.50 percent maximum for the combined Cr, Ni, Mo and V additions, all conforming to AWS A5.18. That generous deoxidizer package is what lets ER70S-6 tolerate mill scale and light rust where ER70S-3 struggles.

• ER70S-6 is the workhorse. Elevated manganese and silicon give forgiving arc behaviour over mildly dirty steel, including light mill scale and surface rust. Choose it when prep is real shop prep rather than laboratory prep.
• ER70S-3 carries less deoxidizer. It lays a cleaner, more fluid bead on bright, properly prepared steel, but it is less forgiving of scale and rust and more likely to show porosity on contaminated metal.
• ER70S-2 has the highest deoxidizer content. It suits TIG work and dirty or rusty steel, but it is prone to slag, so it is a poor fit for a shop that grinds every bead flush.

Shielding Gas: The Blend That Shapes the Arc

The gas protects the hot filler metal and molten pool from the atmosphere, and it also has a real effect on the arc itself.

• 75% argon / 25% CO2 (C25, also sold as M21) is the standard blend for carbon and mild steel. Argon keeps the arc stable and gives clean wetting, while the CO2 adds ionization that helps penetrate thicker material.
• 90% argon / 10% CO2 suits thinner steel and short-circuit transfer. The higher argon content gives better penetration on thin material, a more controllable pool and less spatter.
• 100% CO2 (C1) is the cheapest gas and penetrates well, but it gives a less stable arc, noticeably higher spatter and more fume. An OSHA compliance directive notes that carbon dioxide shielding is known to produce far higher amounts of welding fume, so the fume savings of an argon blend matter in confined or high-production work.
• Pure argon is the choice for aluminum and stainless, where reactive alloying elements would otherwise oxidise. For stainless, argon-dominant mixes such as 98% argon / 2% CO2 or the tri-mix with a small helium addition are typical.

Flow rate matters too. For C25 on mild steel, run roughly 15 to 20 CFH for thin material and 20 to 25 CFH for thicker sections. Too little gas and the arc sees air, which means oxidation and porosity. Too much and you create turbulence that disturbs the puddle and pulls air in around the edges of the shielding envelope.

Material	Wire	Shielding Gas	Notes
Mild steel	ER70S-6	C25 (75/25)	Default for most work
Thin mild steel	ER70S-6, 0.023 in	C10 (90/10)	Cleaner puddle, less spatter
Aluminum	ER4043, ER5356	100% argon	ER5356 is stronger, crack resistant
Stainless	ER308L, ER309L	Argon plus 1 to 3% CO2	308L for 304/308, 309L for dissimilar
Brazing	ER4043, ER5356, ERcuSi-A	Argon or nitrogen mix	ERcuSi-A also suits galvanized steel

For aluminum, the practical difference between the two main filler alloys matters. ER5356 carries higher strength and far better resistance to hot cracking, which makes it standard for structural and repair work; our guide to aluminum MIG welding covers the joint preparation this material really needs. The same rules apply on a vehicle, covered in the alloy wheel welding guide.

Solid Wire Versus Flux-Cored Wire

Solid wire is what you get with a gas bottle and an external shielding gas. Flux-cored wire is a different animal, and it earns its place under different conditions.

Gas-shielded flux-cored wire such as E71T-1C is classified under AWS A5.20. It still needs shielding gas, but the flux core decomposes into a slag that adds deoxidizing elements and lets the arc run at higher amperage with deeper penetration and higher deposition rates than solid wire. It is the general purpose choice for heavier structural steel.

Self-shielded flux-cored wire, classified as E71T-11 or E71T-GS, needs no gas bottle at all. The flux core provides its own shielding, so the welder carries nothing but wire. That single fact makes it dramatically more portable and far better suited to outdoor work in wind, where a gas-shielded gun loses protection the moment the breeze crosses the nozzle. It also produces visible slag that needs chipping and it is stickier to handle than solid wire.

Miller’s comparison of solid wire and flux-cored wire covers the trade-offs in more detail. The rule of thumb is simple: solid wire with mixed gas is your clean, versatile everyday process, while flux-cored earns its place outdoors, on dirty structural steel, or when chasing deposition rate.

Machine Settings: Wire Feed Speed, Voltage, Travel Speed and Gas Flow

Almost every disappointing MIG bead traces back to one of three dials, and the reason settings feel confusing is that most machines do not label them the way an engineer would. Here is the mental model that makes the rest of this guide click into place: on a constant-voltage (CV) machine, wire feed speed sets the amperage, and voltage sets the arc length. Penetration, bead width, spatter, and heat input are all consequences of those two numbers. Get that relationship straight and you can reason your way to a good weld instead of copying settings off the internet and hoping.

Wire feed speed is your amperage dial

A CV machine holds the voltage you dial in and lets the wire speed decide how much current flows. Feed more wire through the drive rollers and the machine raises current to keep pace, which means more heat, faster deposition, and deeper penetration. Slow the wire down and current drops, the bead flattens, and penetration shrinks. AWS describes GMAW as a constant-voltage process for exactly this reason.

For a starting point, Miller’s parameter guidance suggests roughly 1 amp for every 0.001 inch of material thickness, and for 0.030-inch wire, about 2 inches of wire feed per minute for every amp of current. So a 0.250-inch plate (roughly 250 amps) lands near 500 ipm of 0.030 wire. Larger wire feeds slower for the same current because each inch of it carries more amps, which is why the table below uses 0.035 wire on the thickest row.

Starting points by material thickness

Material	Wire	Wire feed speed	Voltage	Transfer
18 ga (0.035 in)	0.023	100–150 ipm	15–16 V	Short circuit
16 ga (0.060 in)	0.023 / 0.030	150–200 ipm	16–17 V	Short circuit
3/16 in	0.030 / 0.035	200–275 ipm	17–19 V	Short circuit
1/4 in	0.035	250–325 ipm	18–20 V	Short circuit

All four rows assume C25 shielding gas at roughly 15–25 CFH, depending on nozzle diameter and material. ESAB identifies 75–82% argon with 18–25% CO2 (M21/C25) as the standard blend for most carbon and mild steel MIG work, and it is the right default for everything in this table. If you are still choosing a cylinder and regulator, our guide to welding gases, cylinders and safety breaks down the options.

Voltage: the arc length dial

Raise the voltage and the arc physically gets longer, which widens the bead and changes how the pool flows. Both ends of that dial produce recognisable symptoms.

• Voltage too high: poor arc control, inconsistent penetration, and a turbulent, churning weld pool. The arc hovers, the puddle is disturbed, and the bead edges wander.
• Voltage too low: the arc starts hard, spatter flies, the bead comes out convex, and the toes of the weld do not tie smoothly into the base metal.

Travel speed: the variable most people forget

Wire feed speed and voltage get all the attention, but the speed at which you move the gun is a third heat control, because heat input is roughly amps multiplied by volts divided by travel speed.

• Travel too fast: a narrow, convex bead with inadequate tie-in and insufficient penetration. The puddle does not have time to wet the edges of the joint.
• Travel too slow: excess heat, a wide bead, poor penetration, and burn-through on thin material. The puddle sits long enough to grow wide and dished rather than drive downward.

Stickout, or electrode extension

Stickout is the distance from the contact tip to the work. Forney’s quick-start guide recommends 1/2 inch plus or minus 1/8 inch for MIG. Longer extension adds electrical resistance in the wire, so the same feed speed delivers less current, and the arc is further from the shielding coverage, which raises the risk of porosity. Too short and you get stubbing, erratic starts, and a worn contact tip. Set it once, glance at it often.

Positional work changes the numbers

Vertical and horizontal welding cannot use flat-position settings. ESAB advises reducing wire feed speed and voltage relative to flat position and prefers short-circuit or pulse transfer out of position, because gravity is already dragging the puddle downhill and lower heat keeps it where you put it. Our all-position consumable guide covers the wider topic of filler metals that tolerate every position.

Synergic and pulse MIG

Synergic machines store a program that ties wire size, material and thickness to a proven wire feed speed and voltage combination, and many will compensate for stickout as you move. Pulse MIG goes further, modulating current at high frequency to hold a short, heat-controlled arc that produces a slag-free, spatter-light bead. That is why it is the go-to for stainless, thin material and out-of-position work, and for aluminum, where spray transfer demands tight control (see our complete aluminum MIG guide).

One last habit pays for itself quickly: write down the material, thickness, wire, gas, flow rate, wire feed speed, voltage and travel speed for every weld that goes well. Settings that worked on a specific machine, on specific stock, are worth more than any chart.

Welding Technique: Gun Angle, Work Angle, Arc Placement and Travel Control

Two welders can run identical wire, identical gas, identical settings on the same joint, and one will produce a clean, fully penetrated weld while the other gets a lumpy bead with undercut on one toe. The machine didn’t change. The hands did.

That is why technique is not the soft part of good MIG welding. It is the part that decides whether your settings land anywhere useful. AWS tracks improper technique as a common root cause behind both incomplete joint penetration and incomplete fusion, the two defects that quietly fail a lot of parts that otherwise look fine.

Gun angle: about 15 degrees from vertical

Hold the gun tilted roughly 15 degrees off vertical, pointing the direction you are traveling. AWS recommends this angle because it does three useful things at once: it keeps the arc properly shielded by the gas stream, it throws spatter behind the bead where it won’t coat your work, and it gives you a clear line of sight into the puddle.

Tilting further back is tempting because it “looks more comfortable.” Resist it. Push the gun too far over and you expose the arc and puddle to the atmosphere, which pulls in oxygen and nitrogen and produces porosity. The puddle also gets harder to see, and the arc starts to push rather than direct.

Work angle: where the torch sits relative to the joint

Gun angle is how the torch is tilted. Work angle is where the torch is positioned in relation to the joint geometry. They are different things, and confusing them is a common source of stubborn undercut.

For a square butt joint, the work angle should be zero. The torch points straight into the joint. Angle it off to one side and the arc force shoves the puddle across the root, chasing the molten metal toward one side and undercutting the other. You get a bead that looks like it landed slightly off-center on one edge, and the same cause shows up as incomplete penetration because the puddle no longer sits over the root opening.

For a fillet weld, the work angle is roughly 45 degrees, with the torch positioned so the nozzle fits into the corner of the joint. This is the single most common early mistake. Push the torch into the corner at anything like a 90-degree position and you get a narrow bead with no sidewall fusion, plus undercut where the arc force has nowhere good to push the metal.

Joint type	Work angle	What goes wrong otherwise
Square butt	0 degrees (torch straight into the joint)	Puddle pushed off root, undercut on one toe, incomplete penetration
Fillet	About 45 degrees into the corner	Narrow bead, weak sidewall fusion, undercut along one leg
Lap	Work on the edge, torch just off the corner	Arc force pushes molten metal off the edge, poor edge tie-in

Arc placement: keep it on the leading edge

Here is the principle that ties technique together. Direct the arc at the leading edge of the puddle, meaning the edge of the molten pool that is advancing toward unwelded metal. The arc force then drives molten metal forward into the cold joint, which promotes fusion and flattens the bead profile.

Put the arc in the middle of the puddle or trailing behind it and the arc force works against you. Molten metal gets pushed back toward you instead of forward, the puddle shrinks, you lose the ability to see the edges of the joint, and you are on a fast track to lack of fusion.

This is also why viewing the puddle matters. If you cannot see where the pool edge sits in relation to the joint, you cannot place the arc on that leading edge. The answer is not a bigger helmet lens. It is a good welding helmet setup with the right shade and a clear lens, plus cleaning your nozzle so you are not looking at a wall of spatter.

Push technique versus drag technique

Both are legitimate. They are simply different tools for different situations, and pretending one is always better is how beginners end up fighting their own gun.

• Push (drag) technique: the torch leads the joint, angled back roughly 15 degrees, pushing the puddle forward. This gives better penetration and a flatter, wider bead because the arc force drives heat into the base metal. The cost is more spatter, a more visible spray of metal, and a slightly harder view of the puddle.
• Drag technique: the torch trails the joint, angled into the direction you came from. You get a narrower, more convex bead with a slightly higher reinforcement, less spatter, and a clearer view of the puddle ahead of the arc. Penetration is lower, which matters on thicker material or on a joint with a tight root opening.

Most carbon steel work is pushed. Drag is worth keeping in your back pocket for thinner material, for working in tight spaces where spatter is a problem, and for joints where you want to watch the puddle closely.

Stringing versus weaving

Stringing beads means moving the torch steadily along the joint without weaving, letting the crater fill and lay down behind the arc. It is faster, and on thinner material it is often the right call.

Weaving means moving the torch side to side to widen the bead. This is where undercut and lack of fusion usually begin, and AWS specifically instructs welders to pause on the groove side walls when weaving. That pause is what heats the sidewall to fusion temperature. A fast side-to-side sweep without dwelling on the walls leaves the puddle swinging around the joint without ever melting the edges properly, and you get a wide, impressive-looking bead that is not actually bonded to the sides.

Travel speed: the variable most operators forget

Your machine settings can be perfect and your weld still bad if your hand speed drifts. AWS instructs welders to ensure consistent travel speed to prevent undercuts or excess spatter, and ESAB points out that varying travel speed changes heat input even when machine settings are fixed.

Think about what that means physically. Heat input is a function of power and time over distance. Your machine delivers power on a cycle. Your hand delivers the time. Speed up and you deposit the same wire over a longer distance, so each inch of joint receives less heat: narrower bead, less penetration, higher chance of undercut because the puddle has less time to wet out the toe. Slow down and you pile up heat in one spot: wide bead, poor penetration, and on thin material, burn-through.

Practical approach: pick a travel speed you can sustain for the whole joint, and let the machine settings adjust around it. If the bead is too narrow, you are moving too fast for the amperage you have set. If it is too wide and domed, slow down or turn up the wire feed speed.

Contact tip-to-work distance and interpass cleaning

Hold a consistent contact tip-to-work distance through the whole weld, typically around half an inch of stickout from the nozzle. If the tip-to-work distance drifts, the arc length changes even though you never touched a knob, which changes voltage, which changes bead shape. Watch the length of your arc instead of the length of your stickout, since arc length is what actually matters.

Between passes, chip and clean the weld. Grind off the pass you just made, brush away spatter, and remove any scale. Every bit of slag or mill scale left in the toe of the joint is a place the next bead cannot fuse to properly.

Finally, look at your own work. AWS D1.1 practice and the equivalent structural codes encourage a welder’s own visual inspection of their weld, because the person holding the torch can see things a plan reviewer never will. You should be able to look at your bead and say what the arc did and why.

That self-read is a skill, and like any skill it improves with deliberate practice. Work through the process on scrap coupons of the same material and thickness you actually build with, and keep a small log of what you changed. If you want a broader look at where technique meets process selection, our guides on aluminum MIG welding and aluminum flux core welding cover the technique adjustments that non-ferrous work demands.

Joint Preparation, Fit-Up and Cleanliness: The Unglamorous Half of a Good Weld

Here is the unglamorous truth about MIG welding: most failed beginner welds fail before the arc is struck. A beautiful bead over a dirty, mismatched joint is still a weak bead. The machine settings you learn in any good MIG welding guide can only do so much when the metal underneath is oily, misaligned, or covered in mill scale.

Clean the Metal Properly, and in the Right Order

Cleaning is a sequence, not a single swipe with a rag:

1. Degrease first. Wipe or scrub the joint area with a solvent such as acetone or mineral spirits. Oil and grease cause porosity because they break into gases as the arc heats them, and those gases get trapped in the solidifying weld metal.
2. Remove paint and mill scale next. A grinder or flap disc takes off paint, primer, and the tough blue-black scale on hot-rolled steel. Scale is dangerous because it flakes: a flake that lands in the puddle blocks fusion or traps gas, and you find it later as a pit in the finished joint.
3. Knock off loose rust last. A wire wheel or flap disc removes light surface rust without leaving hard scale behind.

This is where ER70S-6 earns its place as the industry default carbon steel wire. Its elevated manganese and silicon act as deoxidizers, so the weld tolerates light mill scale and light rust better than ER70S-3. That tolerance has limits: scale flakes, oil, grease, and moisture are a different category entirely, and no filler metal rescues them.

Watch the consumables too. Lubricant picked up in the feeder or liner from an over-greased drive roller is a documented cause of excessive spatter, and oil or moisture on the wire carries into the puddle. ITW’s own troubleshooting guidance lists a dirty workpiece as a cause of excessive spatter and incomplete fusion, and recommends removing all grease, oil, moisture, rust, paint, and undercoating before welding.

Fit-Up: Gap, Root Opening and Alignment

Fit-up determines whether your machine settings work for you or against you. Three things matter:

• Root opening. A square butt joint with zero gap gives the arc no room to reach the bottom of the joint, so you get a cold, incomplete joint. A gap roughly equal to the wire diameter, or a groove with a root face, gives the puddle somewhere to go.
• Alignment. Misaligned parts tax the welder and cause rework: one side gets starved, the other gets buried, and the joint ends up out of square. ER70S-6 joins slightly misaligned pieces more easily than ER70S-3, part of why shops reach for it on less precise work.
• Clamping and tacking order. Tack before you clamp hard, or clamp, tack, check, then adjust. A straightedge across the joint tells you in two seconds what a cap full of weld metal will not.

Tacks Count as Welds

Beginners treat tacks as throwaway. Codes do not. AWS D1.1 requires a tack weld to be reasonably uniform in appearance and free of overlap, cracks, and undercut exceeding 1/32 inch (1 mm), with no porosity visible on its surface. So keep them small, evenly spaced, and clean. A sloppy tack is a defect you will grind out or, worse, weld over and live with.

Matching Joint Design to Groove Prep

Joint	Prep notes
Butt	Square gap for thin stock; groove with root face or a V-groove for thicker material.
Lap	No groove needed; fit tightly and weld both edges where load requires.
Tee	Fillet weld; ideally a small radius in the corner for a smoother, stronger toe.
Corner	Fillet weld; keep parts square and use a backing bar for thin material.
Edge	The weakest joint. Needs very tight fit and consistent heat to avoid burn-through.

Galvanised Steel Needs Its Own Plan

Zinc creates two problems: it contaminates the weld, and when the arc vaporises it you get zinc oxide fume. OSHA’s welding safety fact sheet notes that zinc oxide exposure can cause metal fume fever, with flu-like symptoms such as fever, chills, nausea, and chest pain. The accepted options are to grind the zinc off before welding, or to MIG braze with a copper-silicon wire such as ERCuSi-A (CuSi3), which wets the zinc without melting the coating.

Welding Outdoors

Wind ruins shielding gas coverage, and there is no code workaround. Use a wind screen and increase gas flow. AWS D1.1 Section 5.12 is unambiguous: welding is not performed when surfaces are wet or exposed to rain, snow, dust, or high wind. AWS’s GMAW overview puts it plainly: the shielding gas is doing protective work, and it cannot work in a gale.

None of this is glamorous. It is also where a solid weld is won or lost, before the trigger is pulled.

How to Read Your Own Weld: What Good Looks Like, Defect by Defect

The most useful skill for any MIG welder isn’t cranking up the amps, it’s learning to read your own bead. Machine settings can be copied, but an honest eye will fix far more issues than a new power source ever could.

What a Good MIG Weld Looks Like

Before you start hunting for flaws, know what you’re aiming for. For most structural work, a sound MIG weld should meet these points:

• Consistent profile: A slightly convex bead with a smooth, even ripple that matches your travel speed.
• Clean toes: The edges should blend into the base metal with no sharp notch or ledge.
• Even width and reinforcement: The bead shouldn’t balloon in one spot or taper off in another.
• Minimal spatter: A small amount is normal, but excessive spatter is usually a symptom of another issue.
• Sound fusion: What you see on the surface should tie into the joint. Appearance is only a clue, not a guarantee.

A plainer, fully fused bead will almost always be stronger than a proud, ropey one. Looks alone won’t carry a weld, fusion will.

Defects to Watch For

When inspecting, don’t look straight down at the crown. Check the toes first, as that’s where most rejectable flaws like to hide.

Undercut

Undercut is a groove melted into the base metal at the toe of the weld, leaving a thinner section behind it. Common causes include too much heat, travelling too fast, a poor work angle, or welding over dirty metal. For code work under AWS D1.1, undercut has strict limits: for material 1/4 inch and thicker, it’s capped at 1/32 inch under static loading, and 1/16 inch for thinner material under static loading. For the most restrictive dynamic loading applications, it can be zero. Most importantly, cracks and lack of fusion are rejected outright regardless of size.

Porosity

Porosity shows up as pinholes or clusters of holes throughout the bead. Of particular concern is piping porosity, described in AWS practice as elongated porosity with its major dimension lying approximately normal to the weld surface. It’s almost always caused by atmospheric contamination, such as poor shielding gas coverage, excessive stickout, gas leaks, moisture, or oil on the base metal or wire.

According to the FHWA Bridge Welding Reference Manual, most discontinuities found by magnetic particle testing, such as undercut and cold lap, are also discoverable by a close visual inspection. Tight cracks tend to be the exception, which makes a disciplined visual check a genuine quality gate.

Convexity, Overlap, Cold Lap, and Cratering

Excessive convexity builds up far taller than it is wide, creating excess reinforcement with no added joint strength. While some reinforcement is acceptable under AWS D1.1 weld profile and visual inspection acceptance criteria, overly ropey beads can create stress risers and are often a sign of too little travel speed or voltage set too low.

Overlap is when molten weld metal rolls over onto cold base metal without fusing to it. Cold lap is its more insidious form, where the toe appears tied in, but lack of fusion sits underneath the rolled edge. Both are usually caused by poor arc placement at the leading edge of the puddle or welding too cold.

Cratering is the depression left at the end of a weld from abruptly extinguishing the arc. These craters can become crack initiation points, especially on thicker or highly restrained joints. Always fill your crater before breaking the arc.

Your Self-Inspection Routine For Every Pass

1. Prep first: Confirm your joint is clean before striking an arc. For certain alloys, prep is just as critical as settings. For example, when working with aluminum, taking the time for thorough prep using the correct aluminum cleaner for welding can be the difference between a sound bead and porous scrap.
2. Check the toes: Scan both toes with a good light. Look for any notch, groove, or rolled edge. If your eye catches a sharp transition, investigate it further.
3. Evaluate profile: Look for uniform width, an even ripple, and consistent reinforcement from start to finish. Pay extra attention to starts, stops, and tack areas, as these are the most common spots for craters and cold lap.
4. Scan for surface flaws: Check for pinholes, excessive spatter trails, undercut, overlap, or cracking along the bead. Adjust your viewing angle if you’re unsure about the toe, rather than just looking straight down at the crown.
5. Verify for the next pass: For multi-pass welds, inspect and clean between passes. Slag, spatter, and loose contaminants can compromise the next bead no matter how perfect your parameters are.

Reading your weld is a muscle. The more honestly you critique each coupon, the quicker your brain will connect a flaw to its fix. That feedback loop is what turns a decent bead into a consistently good, sound MIG weld.

Troubleshooting: A Symptom-to-Cause Checklist for the Defects You Will Actually Meet

Most MIG problems are not mysteries. They repeat. Heavy spatter, a bead peppered with pinholes, a puddle that will not sit still: each points to one of a short list of causes. The real difficulty is that beginners change four things at once, get a slightly different bead, and blame the machine. It is almost never the machine.

Troubleshooting works far better as an order of operations than as a guessing game. Rule out the cheap, invisible causes first, because they do the most damage and the least warning.

Work the diagnostic order

1. Shielding gas and atmosphere. Check the flowmeter on the cylinder you are actually welding from. Listen for leaks at the regulator, hose, and gun body. Confirm the nozzle is not blocked, and check for wind or a fan blowing across the arc. Gas problems masquerade as every other defect on this list.
2. Cleanliness. Degrease the joint and check the wire spool for oil, fingerprints, or a film. Clean metal and clean wire cost nothing to confirm and eliminate a huge share of porosity.
3. Stickout and consumables. Check electrode extension (about 1/2 inch), contact tip condition, liner, and nozzle. These wear out silently and change the arc before anything else does.
4. Parameters. Only then touch wire feed speed and voltage, using the starting points in the Miller parameter guide.
5. Technique. Gun angle, work angle, travel speed consistency, and where you hold the arc in the puddle.

The order is not arbitrary. A dirty liner spatters no matter how the machine is set; a leaking gas hose gives you porosity no matter how clean the metal is. Fix those first and half your “parameter problems” disappear.

Symptom to cause

Symptom	Most likely causes	Corrective action
Excessive spatter	Wire feed speed too high, voltage too high, electrode extension (stickout) too long, dirty workpiece, insufficient shielding gas, dirty welding wire, lubricant pickup from the feeder or liner	Drop wire feed speed and voltage in small steps, shorten stickout, clean the work and the spool, raise gas flow, and strip down the gun cable and liner
Porosity (pinholes, piping)	Insufficient shielding gas at the welding arc, excessive contact tip to work distance, dirty or oily metal, moisture, gas flow too low or too high, leaking gas hose, low argon purity	Restore correct flow, bring the gun closer, degrease and dry the joint, find the leak, and check cylinder purity with your welding gas supplier
Excessive penetration / burn-through	Excessive heat input	Lower voltage, reduce wire feed speed, increase travel speed
Incomplete joint penetration	Improper joint preparation, improper technique, insufficient heat input	Raise wire feed speed and/or voltage, reduce travel speed, improve groove geometry and fit-up
Incomplete fusion	Dirty workpiece, insufficient heat input, improper technique	Select higher voltage and/or adjust wire feed speed, place the bead in the proper locations in the joint, pause on groove side walls when weaving, and keep the wire on the leading edge of the puddle
Bead waviness or ripples	Voltage too low or wire feed speed too high, travel speed too slow	Lower the voltage range and/or reduce wire feed speed, increase travel speed, and weld in small segments while allowing the section to cool

The spatter list is the one most ITW troubleshooting material agrees on, and it is worth memorising because spatter is the earliest visible warning. The Forney quick start guide lists it, and it matches the shop floor.

Use your ears and the puddle as instruments

A good MIG arc sounds like a steady crackle, a little like frying bacon. Change that sound and you have information. A hiss means gas escaping somewhere. Sharp spitting pops mean moisture is reaching the arc, usually condensation on cold steel or a damp spool. A hollow, dull tone usually means voltage is too low for the wire feed speed.

Watch the puddle, not the arc. It should stay small and quiet, roughly the size of a dime to a quarter depending on amperage. Grow into a lazy, bulging lake and your travel speed is too slow. Shrink to a tight, hard dot and you have too little heat. AWS describes GMAW transfer modes in enough detail to identify which mode your settings are actually giving you.

Change one variable at a time

This rule saves more time than any other. Adjust voltage, run a short coupon, look at the bead, then adjust wire feed speed. Never move both, because you will never know which one fixed it and the next weld on a different thickness will defeat you.

And when a joint refuses to behave on MIG, do not fight it. Some repairs, dirty structural steel, or outdoor field work suit another process better. Why SMAW still wins in many situations is worth knowing, because the best troubleshooting decision is sometimes to change process rather than chase parameters.

Safety and Ventilation: Welding Without Wrecking Your Lungs

Nobody plans to get sick from welding. You clamp a bracket, drop the helmet, and the job takes twenty minutes. The arc vaporises a fraction of the workpiece, and that vapour cools into a microscopic dust you breathe in with every weld. That is welding fume, and understanding it is the difference between a long career and a chronic cough at fifty.

What Is Actually in Welding Fume

Welding fume is not smoke from burning material. It is fine metal oxide particles formed when the arc melts base metal, filler metal and coating, and the heat turns those metals into vapour that condenses into dust. OSHA’s welding fume fact sheet (FS-3647) lists the metals it can contain: aluminium, antimony, arsenic, beryllium, cadmium, chromium, cobalt, copper, iron, lead, manganese, molybdenum, nickel, silver, tin, titanium, vanadium and zinc.

What you are exposed to depends on the alloy. Mild steel gives you iron and manganese, stainless adds chromium and nickel, and galvanised steel adds zinc. The more exotic the material, the longer the hazard list, which is worth remembering when you pull alloy steel welding electrodes for a stainless or chrome-bearing job.

Acute Symptoms and Long-Term Damage

Acute exposure announces itself: eye, nose and throat irritation, coughing, dizziness and nausea. Metal fume fever shows up a few hours after a heavy exposure, with flu-like aches, chills and fever, and usually clears within a day or two. It feels like you caught something. You did not, and the short-term effects are what leave a permanent mark.

Prolonged exposure may cause lung damage and cancers of the lung, larynx and urinary tract. Certain components damage the kidneys and the nervous system. Manganese, present in nearly every steel weld, is linked to Parkinson’s-like symptoms such as tremor, rigidity and slowed movement when exposure is heavy and repeated. You do not have to feel sick for fume to be costing you.

Hexavalent Chromium: The One With Its Own Regulation

Chromium needs its own conversation. When the arc heats stainless steel or any chrome-bearing alloy, the heat oxidises chromium into the hexavalent form, Cr(VI), which is far more toxic and far more mobile than the chromium sitting in the metal. OSHA’s hexavalent chromium standard sets a permissible exposure limit of 5 µg/m³ as an eight-hour time-weighted average, with an action level of 2.5 µg/m³. NIOSH recommends a much lower limit of 0.2 µg/m³ as an eight-hour TWA over a 40-hour workweek, so treat the OSHA number as a ceiling rather than a target.

OSHA’s National Emphasis Program ranks likely Cr(VI) exposures from highest to lowest: thermal cutting, SMAW, GMAW, FCAW, air cutting and gouging, plasma welding, then oxy-fuel welding. Your MIG work sits near the top. Indoor welding without engineering controls produces greater Cr(VI) exposures than outdoor welding, and an OSHA compliance directive notes CO2 shielding gas is known to produce far higher amounts of welding fume, so moving away from CO2 on stainless work is a documented way to cut the load.

Ventilation and Local Exhaust

General ventilation is the baseline: enough airflow to clear the work area. Local exhaust ventilation is what captures fume at the source, and distance matters more than hood size. CPWR field research found LEV hoods placed two to three feet from the weld far less effective than the several inches of distance desirable for capture. A large hood held at arm’s length is close to useless; a small hood held near the arc does the job.

Confined Spaces and Asphyxiation

Ventilation also keeps you alive in the wrong place. Argon, helium and carbon dioxide displace oxygen, so a leak in an enclosed space can drop oxygen levels without warning, and carbon monoxide can form in enclosed spaces as a second asphyxiation hazard. Welding inside a tank or vessel is confined space work: permit, monitor and attendant, not just a fan pointed the wrong way. The Air Gas welding supply guide covers cylinder chain, cap and storage basics that prevent a lot of avoidable trouble.

Personal Protective Equipment

A welding helmet with a shade matched to the process and amperage is the baseline, and grinding or slag chipping still calls for eye protection under it. Respirators are the next level up, and they supplement ventilation rather than replace it: when monitoring puts you above the action level, or you are welding stainless without local exhaust, a properly fitted respirator is the answer. The standards: OSHA 1910.252 for welding and cutting, 1910.1026 for hexavalent chromium, 1926.1126 on construction sites, 1910.134 for respiratory protection, and 1910.1200 for hazard communication and safety data sheets.

A Practice Plan, Buying Checklist, and What to Do Next

Knowing the theory is only half the battle for good MIG welding. The real progress comes from a structured plan of practice, the right equipment for your workload, and knowing when to seek formal training. With that in mind, here’s a realistic path to take from learning to qualifying your work.

Practice With a Purpose

The fastest way to improve is to practice on scrap that matches the work you actually do. If you mostly fabricate from 3/16 inch mild steel, run your coupons in that exact thickness and material. Heat sink, coatings, and surface condition all change how the puddle behaves, so matching your test metal to your project is key.

To build skill without jumping to every variable at once, work through these positions in order:

1. Flat butt (1F): Focus solely on a straight bead, consistent stickout, and travel speed.
2. Horizontal butt (2F): With the plate turned vertical, you’ll need to actively control the puddle against gravity.
3. Flat fillet (1F): Put your work angle, gun angle, and leading edge placement to use on a 90 degree joint.
4. Vertical-up fillet and horizontal fillet: These two positions are the most common in shops, and the ones that expose the weakest setups.
5. A real project: Once the above feels consistent, weld something you need. The added stakes will expose gaps in your technique quicker than any coupon ever could.

Keep a Settings Log

No two machines run exactly the same. Instead of memorizing universal numbers, record what works for your machine, wire, gas, and shop conditions. A simple log per material and thickness will save countless hours of guesswork.

Material & thickness	Wire / gas	Flow (CFH)	WFS	Voltage	Travel	Result
3/16 in. mild steel	0.030 ER70S-6 / C25	20	250 ipm	18 V	~2 in./min	Good, slight convex
1/4 in. mild steel	0.035 ER70S-6 / C25	25	300 ipm	19 V	~2.5 in./min	Burn-through, fixed by slowing down
16 ga. steel	0.023 ER70S-6 / 75/25	15	160 ipm	16 V	~3 in./min	Clean, flat bead

Be just as strict with your failures. Notes like “travel too fast, cold toe” are invaluable, and if you ever need to qualify, this record is exactly the type of information captured on an AWS WPS or PQR form.

Buying Your First Good MIG Machine

You don’t need the most powerful machine on the floor. You need one that fits your most common work. Focus on these points in order:

• Amperage range: A 200 amp machine will cover most hobby and fab work in mild steel. Go higher only if you regularly weld 1/2 inch plate or thicker.
• Duty cycle: Look for at least 40 percent at your intended output for shop use. Weekend work can be more flexible.
• Wire compatibility: Check spool size and diameter range (commonly 0.023 in. to 1/16 in.). Miller advises choosing a wire size that suits your most welded thicknesses to limit changes, with 0.030 in. or 0.035 in. being the most versatile for plate.
• Flux-core capability: If you weld outdoors, make sure your gun, drive rolls, and polarity can run self-shielded flux core. For a breakdown of when to reach for it over other processes, see the comparison for the advantages of stick welding.
• Synergic features: Helpful while learning, but never at the cost of a stable arc or reliable feeding.
• Consumables: Your contact tip and liner must match your exact wire diameter. A mismatch is one of the most common causes of poor feeding and spatter.

If aluminum ever enters your work, its prep and feeding demands are entirely different. For that, start with aluminum MIG welding: the complete practical guide and pair it with how to prep aluminum right.

Training and Certification

Self-practice will get you far, but structured training makes your skills portable. The AWS education catalogue has online and in-person courses, webinars, technical standards, and interactive modules to build from your current level.

Two paths are worth knowing: AWS SENSE helps you judge the quality of a welding school’s curriculum, and AWS Train the Trainer references hands-on setup for GMAW-S and spray transfer on carbon steel using the instructional procedures WPS AWS EDU GMAW-01 and GMAW-02. For code work, the most recognized route is through the AWS Certified Welding Inspector path and AWS D1.1, which defines welder performance qualification tests and their acceptance criteria. If possible, practice for those D1.1 plates at a local trade school to avoid costly retests.

What to Do Next

Good MIG welding isn’t defined by your machine’s peak amperage. It’s defined by clean metal, matched parameters, disciplined technique, and an honest inspection after every pass. The machine executes your decisions, not the other way around. Start on scrap, change one thing at a time, log every result, and let your own data teach you. That’s the simplest and most reliable plan to follow from here on out.

Frequently Asked Questions

What is the best MIG wire for most home and shop projects?

For carbon and mild steel, AWS ER70S-6 solid wire is the industry default because its higher manganese and silicon deoxidizer content tolerates light mill scale and surface contamination better than ER70S-3. Pair it with a 75% argon / 25% CO2 mix for general work, and switch to gas-shielded flux-cored (E71T-1C) for heavy structural fabrication or when appearance matters less than deposition rate.

Do I need shielding gas if my machine can run flux-core wire?

Only if you use gas-shielded flux-cored wire. Self-shielded flux-cored wire such as E71T-GS or E71T-11 generates its own protective gas from the flux core and needs no bottle, which makes it the better choice for outdoor work in wind. The trade-off is more spatter and slag to chip off compared with solid wire.

Why are there so many holes in my MIG weld?

Porosity almost always points to shielding or cleanliness. Check gas flow first — too little lets the atmosphere attack the pool, too much can swirl air into the arc. Then clean the base metal and the wire, check for oil or lubricant picked up in the feeder, and verify the contact tip-to-work distance is not too long.

What is the right contact tip-to-work distance (stickout) for MIG welding?

About 1/2 inch ± 1/8 inch for most solid-wire work, extending it modestly for thicker material. Longer stickout raises the wire’s electrical resistance, which lowers current at a given feed speed and reduces penetration, and it can promote porosity by letting the arc stretch beyond the gas coverage.

Is MIG welding safe to do indoors in a small garage?

It can be, but only with proper ventilation. Welding fume contains metal oxides that can cause irritation, metal fume fever, and long-term lung and cancer risk, and OSHA notes that CO2 shielding produces significantly more fume. Open the door, use a fan or local exhaust vent that captures the fume at the source, and never weld in a closed space without airflow.

Can I MIG weld galvanized steel?

Yes, but grind the zinc coating off the joint or use a MIG-brazing wire such as ERCuSi-A instead of standard steel wire. Welding over zinc produces zinc oxide fume, the classic cause of metal fume fever with flu-like symptoms and chest pain.

What is the difference between short-circuit transfer and spray transfer?

Short-circuit transfer repeatedly shorts the wire tip into the pool, producing a smaller, cooler bead with less spatter that suits thin material and out-of-position work. Spray transfer melts the wire as droplets, delivering more heat and deposition for thicker sections and flat welding.

How do I fix undercut in a MIG weld?

Undercut comes from too much heat, too little voltage, too much travel speed, a wrong work angle, or dirty metal. Raise voltage slightly, slow your travel, and keep the torch work angle aligned with the joint so the arc is not pushing the puddle away from the toe. Keep the joint clean and grind out any undercut that exceeds the code limit before re-welding.

Can I tell if a MIG weld is good just by looking at it?

Partly. Close visual inspection reliably catches surface defects like undercut, porosity, overlap, and poor profile, which is why welding codes make visual inspection mandatory. But internal problems such as lack of fusion, internal porosity, and buried cracking only show up on radiographic or ultrasonic testing, so appearance alone is not proof on critical or code work.

How do I know if my machine is set up correctly?

Run a short test weld on the same material and thickness as your actual job, then judge four things: arc sound (a smooth crackle is right), bead width and profile, the tie-in at the toes, and spatter level. Write down the wire feed speed, voltage, gas flow, and travel speed that produced it in a log, then adjust one variable at a time from that baseline.