MIG Welding Galvanised Steel: A Practical Guide

A fabricator fires up the MIG set on a hot-dip galvanised gate hinge, and thirty seconds later the bead is covered in ugly grey lumps, the gun tip is crusted with zinc, and the shop smells like a chemistry lab. Nothing is wrong with his machine or his technique on bare steel — the problem is that the zinc coating boils away long before the steel underneath starts to melt. Once you understand that single fact, MIG welding galvanised steel stops being guesswork and starts being a procedure.

Can You MIG Weld Galvanised Steel? The Short Answer Up Front

Picture a typical fabrication shop on a Tuesday afternoon. A fabricator is MIG welding galvanised steel, specifically a hot-dip galvanised gate hinge, the kind that came out of the galvanising kettle with that familiar dull grey crystalline finish. Thirty seconds in, he steps back to find a bead covered in ugly grey lumps, a gun tip crusted with zinc deposits, and a sharp, acrid smell hanging in the air.

Nothing is wrong with his machine. Nothing is wrong with his technique on bare steel either. The problem is that the zinc coating boils away long before the steel underneath starts to melt. Once you understand that single fact, MIG welding galvanised steel stops being guesswork and starts being a procedure.

So, the short answer up front: yes, galvanised steel can be MIG welded, and it is routine work in structural steel, sheet metal fabrication, HVAC ductwork, handrail and balustrade, repair shops, and field maintenance. The American Galvanizers Association (AGA) is equally clear that galvanised steel can be welded without removing the zinc coating, provided special procedures are followed and the work is always done in a well-ventilated location.

What the standards body does not do is prohibit it. The American Welding Society treats welding zinc-coated steel as a qualified specialty rather than a banned practice, with the reference specification being AWS D19.0, Welding Zinc Coated Steel. That document, along with AWS/ANSI Z49.1, Safety and Cutting in Welding, which sets the safety and health requirements for the work, gives fabricators a recognised framework instead of guesswork. A procedure written to D19.0 is a procedure a code inspector and a client will accept.

Two Routes, and You Pick One Before the Torch Comes Out

Almost every galvanised welding job falls into one of two buckets:

1. Remove the zinc before welding. The zinc is ground or burned back from the weld zone, and the weld is made on clean bare steel. This is the AWS-preferred approach for structural work because it gives you predictable penetration, a clean bead and a repeatable procedure. The cost is coating you will have to restore afterwards.
2. Weld straight through the coating. A controlled burn-off technique where the arc is slowed and the heat adjusted so the zinc vaporises ahead of the weld pool. This is the method of choice for field repair, on-site connections, retrofit work and anything where stripping the coating is impractical or would damage the part.

Route	Best for	Core idea
Grind back the zinc	Structural steel, code work, production fab shops	Weld on bright bare steel, then repair the coating
Burn off through the coating	Field welds, sheet metal, ductwork, handrail, repairs	Slower travel, more heat, zinc burns off ahead of the pool

Which Arc Process Fits the Thickness?

On process selection, the AGA offers useful straight advice: GMAW, better known as MIG, is particularly suited to thinner material, under 1/2 in (13 mm) thick. Above that thickness, SMAW, or stick welding, is generally the better answer, mainly because the flux coating on the electrode helps manage the vapour and spatter that come off the zinc, and the slower travel speed gives the zinc time to burn off. Our guide to why SMAW still wins on heavy coated work covers that angle in more depth.

One more piece of shop reality worth repeating: hot-dip galvanising is applied after fabrication for most structures, but field welding is unavoidable when the final structure is too large to fit in the galvanising kettle. The AGA guidance on welding before hot-dip galvanising covers that decision, and it is worth reading before you pick a process.

What This Guide Covers

Over the next sections we work through the whole job: the zinc physics that makes coated steel behave differently from bare steel, the fume and health side of the work including zinc oxide limits and metal fume fever, the AWS coating removal distance, wire and shielding gas selection, machine settings, the burn-off technique itself, troubleshooting for porosity, spatter and poor fusion, and a fair comparison of MIG, stick and TIG.

If you are still dialling in the machine, our notes on welding equipment and setup and how shielding gas works are worth a read first. And if you want the fuller picture of the process, the AGA’s welding galvanised steel page is the reference the industry actually uses.

Why Galvanised Steel Fights Your Torch: The Zinc Physics

Here is the fact that explains nearly every strange thing about MIG welding galvanised steel: the zinc coating boils away long before the steel underneath starts to melt. Slower travel speed, wider root gaps, constant nozzle changes, all of it falls out of one temperature comparison.

Zinc is a low-melting, low-boiling metal. According to the American Galvanizers Association’s zinc metal properties data, pure zinc melts at 419.5 °C (787.1 °F) and boils at 907 °C (1664.6 °F) at atmospheric pressure. Carbon steel, by contrast, needs roughly 1500 °C to reach its melting point. The gap is enormous: when a GMAW arc lands on a galvanised surface, the coating flash-vaporises into the arc while the base metal is still a solid, red-hot lump barely beginning to soften.

Two less obvious numbers sharpen the picture. The latent heat of vaporisation of zinc is 1.782 MJ/kg, so every gram that boils carries away a fixed chunk of energy from your arc. A heavy coating behaves like a heat sink, soaking up energy you would rather spend on the joint. And the vapor pressure of liquid zinc at its melting point is only 1.39 × 10⁻¹ mm Hg, which shows how hard the zinc is trying to escape into the arc the instant it turns liquid. It leaves the coating whether you are ready for it or not.

What that physics does to your weld

1. Porosity and the popcorn bead. The vapourising zinc has to go somewhere, and the easiest path is down into the molten pool. Trapped bubbles leave a wormy, cratered “popcorn” bead that looks like the machine went wrong. It is not a machine fault. It is zinc gas arriving faster than the pool can outgas it.
2. Reduced penetration. The American Galvanizers Association is blunt about this: “Weld penetration depth is decreased when welding galvanized steel,” and larger gaps must be provided for butt welds. Vapourising zinc displaces molten metal, so the pool runs shallower than on bare steel. Butt joints need a wider root gap and a steady side-to-side torch motion for uniform fusion.
3. Spatter that scales with coating thickness. AGA notes that “the formation of spatter particles is directly proportional to the thickness of the zinc coating,” and that spatter is greater for hot-dip galvanised steel than for continuously galvanised sheet. Boiling zinc droplets are flung out by arc forces and land as hard grey lumps on the bead, the plate and your gun.
4. The crust on your nozzle. Zinc vapour that misses the pool condenses on colder surfaces nearby: gas cup, contact tip, workpiece. That crust chokes the nozzle, narrows the gas shield and destabilises the arc, which makes the spatter worse. A self-feeding problem, and the reason nozzle changes belong in the plan rather than in the toolbox.

Coating type changes the severity

Because spatter and fume scale with how much zinc is present, the coating process matters as much as the base metal thickness. Most shop surprises trace back to a job sheet that said “galvanised” without saying how it was coated.

Coating type	Typical thickness	Spatter and fume severity
Hot-dip galvanised (ASTM A123 / A780 fabrication)	About 3.4 mil average (85 g/m²), often heavier	Highest: heavy spatter, dense fume, constant nozzle fouling
Electro-galvanised (electrolytic, ASTM A879)	Roughly 0.4 to 0.8 mil	Low to moderate: manageable with a normal procedure
Continuous galvanised sheet (ASTM A653, Z275/Z350)	Roughly 0.5 to 0.7 mil per side	Low: usually the easiest coated stock to weld

The heat balance you are really managing

AGA’s practical conclusion is that increasing the heat and reducing the welding speed during GMAW gives the arc more time to burn zinc off ahead of the pool front, which is exactly what you want, because clean steel arriving at the pool means less gas, less spatter and better penetration. Add an argon-CO2 shielding gas blend and the arc runs more stably, the deposits come out smoother, and both spatter and zinc loss drop.

That balance is delicate. Too fast and you trap zinc vapour in the pool. Too slow and you burn through thin sheet, distort the part and spread the heat-affected zone well past the weld. Our guide to welding equipment and amperage settings covers how amperage, wire feed speed and travel speed interact. For now, hold onto the principle: you are trying to give the arc enough dwell time to clear the zinc without giving the base metal enough heat to lose its shape.

The Health Side of the Job: Zinc Oxide Fume, Metal Fume Fever and Cr(VI)

Safety on a galvanised weld is not a disclaimer at the bottom of a procedure. It is a design constraint that changes the shop setup, your PPE, and sometimes whether you should light the arc at all.

The physics from the previous section dictates the exposure. Zinc boils at roughly 907 °C, well below the melting point of the steel beneath it, so the arc vaporises the coating into fine particulate long before the base metal flows. That particulate is predominantly zinc oxide, and it carries its own occupational limits.

Exposure limit	Value	Source
NIOSH REL (fume)	TWA 5 mg/m³; short-term limit 10 mg/m³	NIOSH Pocket Guide, zinc oxide (CAS 1314-13-2)
OSHA PEL (fume)	TWA 5 mg/m³	OSHA Table Z-1
IDLH	500 mg/m³	NIOSH Pocket Guide

Those figures come straight from the NIOSH Pocket Guide entry for zinc oxide: a 5 mg/m³ eight-hour TWA with a 10 mg/m³ short-term limit for fume, an OSHA permissible exposure limit of 5 mg/m³ TWA, and an immediately dangerous to life or health value of 500 mg/m³. That last number is worth remembering, because the margin between a comfortable booth and a genuinely dangerous concentration is not wide.

Metal fume fever: the welder’s flu

The most common acute effect is metal fume fever, a flu-like condition that can hit you hours after a heavy fume exposure. NIOSH lists the documented symptoms for zinc oxide fume as:

• Chills and fever
• Muscle ache
• Nausea and vomiting
• Dry throat and cough
• Lassitude and fatigue
• Metallic taste in the mouth
• Headache
• Chest tightness
• Difficulty breathing (dyspnoea)

The respiratory system is the target organ, and onset is typically delayed, which is why welders blame the wrong thing: a bad joint or an off day. The NIOSH welding fumes PEL page notes that an estimated 30 to 40 percent of welders have experienced metal fume fever in their careers, and that recovery usually requires one or two days away from work.

The contaminant people forget: hexavalent chromium

Zinc is not the only concern on a mixed shop floor. OSHA’s hexavalent chromium topic page identifies welding and other hot work on stainless steel and chromium-containing alloy steels as a major source of worker exposure to Cr(VI). Cr(VI) is a known carcinogen that targets the respiratory system, and it can also affect the kidneys, liver, skin and eyes.

This matters for galvanised work because shops rarely weld only galvanised steel. A crew alternating between galvanised handrail, carbon steel structure and stainless process piping accumulates exposure from every bay, so a “galvanised only” mindset understates the problem.

Layered controls, in the order they work

1. Source control first. The best fume reduction is not making the fume. Grinding the zinc back 25 to 100 mm from the weld zone removes the coating before the arc ever sees it, which is exactly why the AWS-preferred method exists.
2. General and local exhaust ventilation. The American Galvanizers Association is explicit that galvanised welding belongs in well-ventilated locations. On a galvanised MIG run, a dedicated fume extraction setup near the source beats a big fan in the corner of the shop. Capture the fume at the nozzle, not after it has filled the bay.
3. Respiratory protection. Per NIOSH recommendations, N95, R95 or P95 particulate filtering facepieces are appropriate up to 50 mg/m³. Above that, fit-tested powered air-purifying respirators or supplied-air protection are required. An off-the-shelf dust mask is no substitute for a respirator program.
4. PPE that actually protects. Eye and face protection rated for the arc, flame-resistant clothing and leather gloves for hot work. A welding helmet protects your face from radiant heat and UV. It does not filter fume, and airflow through a poorly fitted helmet can pull contaminated air towards the face.
5. Gas and consumable handling. Cylinder securing, correct hose and regulator checks and sensible gas practices matter on any MIG job. The practical reference in this welding gas supply and safety guide covers the basics worth rechecking before a fume-heavy job.

OSHA’s Controlling Hazardous Fume and Gases During Welding fact sheet sets out the same hierarchy, with the caveat that PPE should not be the primary defence when fume can be captured at the source.

Fume control changes the procedure, not just the paperwork. On a structural connection in heavy hot-dip coating, grind the zinc back and the exposure problem largely disappears. On sheet metal or a field repair where removal is impractical, burn-off technique has to be paired with real extraction, correct respiratory protection, and the knowledge that you may face a metal fume fever headache by the end of the shift.

I’ll rewrite this section to fit the 560–840 word range while keeping the heading, facts, and links intact.

Method One: Removing the Zinc Before You Light the Torch

If you want the cleanest, most predictable weld you will ever put on galvanised steel, this is the method to use. It costs time, coating material and grinding consumables. It buys you a normal bead, a normal procedure and a normal inspection. For structural work, this is not a fallback. It is the standard.

The American Galvanizers Association is blunt about this. Its guidance on welding galvanised steel points fabricators to AWS D19.0, Welding Zinc-Coated Steel, and the core rule is simple: welds of galvanised steel should be made on areas free of zinc. In practice, the zinc coating should be removed at least 1 to 4 in (2.5 to 10 cm) from either side of the intended weld zone, and on both sides of the piece.

Read that rule twice, because the second half is the part people miss. The strip has to be clean on both faces of the steel. Zinc creeping into the weld from the far side of a plate will still contaminate the bead, cause porosity, and leave a defect that fails inspection.

Why the Grind-Back Zone Exists

The 1 to 4 in band is not arbitrary. Zinc vapour travels. It boils off in the arc, drifts and gets drawn along the plate by weld heat. Weld a coated edge just 1/8 in from a clean zone and you can still find zinc in the bead. Weld on truly bare steel and the zinc cannot reach the weld metal at all, because there is none within reach.

On hot-dip galvanised parts this matters more than most people realise. Hot-dip coatings typically run around 3.5 mils (85 microns) or thicker, a substantial layer of metal compared with the thin electro-galvanised or pre-galvanised sheet used in HVAC and fabrication work. Heavier coating, more zinc, more reason to remove it before welding.

The AGA also lists its own knowledge base article, Can You Weld Galvanized Steel?, covering the same ground and directing fabricators to AWS D19.0 for procedure details. Use both. The AGA explains the philosophy, the AWS standard carries the numbers your inspector will want to see.

Three Ways to Get Zinc Off

• Grinding back. The preferred method. Mechanical removal with an angle grinder, flap disc or abrasive blast. The joint ends up chemically clean of zinc in the weld zone.
• Burning the zinc away. Heat the coating with a torch until it burns off before welding. Faster on big areas, but the operator is deliberately making the same zinc fume the welding process is meant to avoid.
• Pushing it back. Use a torch or grinder to move the zinc bead out of the weld area so the joint is bare while coating further along stays intact.

Method	How it works	Strengths	Weaknesses
Grind back	Angle grinder, flap disc or abrasive blast removes coating 1 to 4 in (2.5 to 10 cm) each side of the weld zone on both faces	Cleanest joint, easiest to verify, best for structural and code work	Slowest, costs consumables, leaves bare steel that must be repaired
Torch burning	Heat coating with a torch until it burns away	Fast over large surfaces, minimal equipment	Generates zinc fume before welding even starts, harder to control the exact stripped width
Push back	Melt or grind the zinc bead away from the joint with a leading edge	Keeps most of the coating, useful for field repair	Relies on torch angle and speed, less repeatable, still needs a clean zone to weld
Leave it and burn off during welding	Weld straight through the coating with a slow, hot burn-off technique	Fastest, no prep, good for sheet, ductwork and remote work	Porosity risk, heavy spatter, fume exposure, harder to inspect, not preferred for structural work

Step-by-Step Grind-Back Procedure

1. Identify the coating first. Know whether you are looking at hot-dip galvanising, electro-galvanising or pre-galvanised sheet, and estimate the thickness. Coating type dictates how aggressive you can be and how much you need to remove.
2. Mark the zone. Use a permanent marker and masking tape to outline the strip you will grind, 1 to 4 in (2.5 to 10 cm) from the weld centreline on both sides of the joint and on both faces. Tape keeps the boundary straight and catches overspray.
3. Grinder with a guard. Fit a flap disc or grinding wheel rated for the job and always use the guard. Work at a low angle to avoid gouging the plate.
4. Work in stages. Take light passes and stop periodically. You are after bright bare steel, and you can often feel the change as resistance under the disc drops. This is the step most people rush, and it is where coating survives in the corners and toes of the joint.
5. Feather the edge. Feather the transition from bare steel to remaining coating. A sharp step in the coating creates a notch where the weld pool sits, and it is a natural crack starter.
6. Wire-brush or blast the joint. Clean the prepared zone with a stainless wire brush or light abrasive blast to remove scale and debris, then remove any remaining zinc-rich residue right at the joint edges.
7. Check fit-up and inspect. Verify the joint is genuinely zinc-free before you strike an arc. See the note below.

The Trade-Off Nobody Loves: You Have Made a Bare Spot

Grinding the zinc away is not free. You have sacrificed coating on both faces of the member, and that bare steel will start to rust the moment it meets weather. On a walkway or handrail, that bare band is the first thing a client notices six months later.

The AGA warns about exactly this. Removing zinc coating creates a bare, unprotected area that must be repaired. For hot-dip galvanised work, the stripped zone has to be restored after welding with a zinc-rich touch-up paint or a thermal-spray (metalising) system. You will find the same requirement in the AGA’s guidance on repairing HDG steel. Use a zinc-rich touch-up paint and galvanised steel repair guide to restore the coating properly and match the surrounding finish.

That said, most fabricators find the arithmetic favours grinding. Removing coating is slow and slightly wasteful. Managing the burn-off technique is slow, unpredictable and produces the fumes you spent the whole day trying to avoid. Grind the zone, weld it properly, paint it back. The work is bigger on paper and smaller in practice.

Method Two: MIG Welding Straight Through the Zinc (Burn-Off Technique)

Sometimes there is no option but to weld through the coating. The joint is already on site, the galvanising is finished, the drawing says leave the finish alone, and there is no grinder budget or touch-up kit in the truck. Field repair of handrail and balustrade, on-site structural connections, sheet metal and HVAC ductwork, and most retrofit and alteration work all land here.

The burn-off technique, sometimes called welding through the coating, uses the arc itself to vaporise the zinc immediately ahead of the pool, then deposits clean weld metal into the void the zinc left behind. The American Galvanizers Association lays out the GMAW logic in a sequence worth following.

The AGA’s GMAW Playbook

• Slow your travel speed down. AGA states that GMAW speeds are typically slower on galvanised surfaces, and the reduced speeds give the zinc longer to burn off at the front of the weld pool.
• Add current when the coating is heavy. Increasing the current supplied to the welding electrode may provide sufficient means to burn off zinc coatings of greater thickness.
• Plan for less penetration. Zinc vapour displaces molten metal, so butt joints need a wider root gap than on bare steel. Consistent penetration comes from a side-to-side torch motion when butt-welding in the flat position.
• Expect more spatter. Spatter climbs with CO2 shielding and with coating thickness, because boiling zinc droplets get flung out of the pool.
• Prime the joint before you strike an arc. AGA recommends silicon-, petroleum- or graphite-based spatter release compounds applied beforehand. They reduce spatter adherence so spatter can be brushed off instead of fusing to the coating around the bead.

That step is the one most people skip, and it is the cheapest productivity win on hot-dip work. A thin coat of anti-spatter compound on the faying surfaces and around the joint turns crusty zinc balls into loose crumbs. Our guide to welding supplies and consumables covers what is typically on the shelf.

It Is a Heat Balance Problem

Every parameter in that list comes back to one trade-off, and getting it wrong costs you twice.

This is also why arc length and torch angle matter more than usual. Keep a short arc and a stable work angle, roughly 10 to 15 degrees off vertical with a slight push, so arc force drives the pool forward and vapour is swept away from the bead. Hold the nozzle about 3/8 in (10 mm) off the work. A long, wandering arc lets fumes pool around you and spreads heat into coating you have no need to sacrifice.

The Whip and Weave in the Field

The technique field welders settle on is a whip: a quick forward flick across the coated section, then a slower dwell while the pool advances into clean steel. The whip burns the zinc ahead of the pool; the dwell is the actual weld. It is the fast-flick rhythm welders learn with all-position rod work, repurposed so the flicker lets vapour escape instead of slag.

For flat butt joints, work in the side-to-side weave the AGA specifies, keeping the dwell at the edges of each pass rather than weaving fast and thin. On vertical and overhead fillets, keep the whip short and the pauses deliberate.

There is a hybrid worth knowing: the leading-edge standoff. Some fabricators hold a scrap bar or a small angle grinder body at the leading edge of the joint so the molten zinc bead is pushed back and out of the arc path, letting the arc work mostly on clean steel. It is fiddly and it slows you down, but on a structural member it can save you from grinding the whole area back.

Burn-off welding is not the AWS-preferred route for structural fabrications, and it is not a licence to skip ventilation. Treat it as a controlled process with a qualified procedure, a tested travel speed and extraction in place, not a shortcut around the grind-back rule.

Wire, Gas and Machine Settings That Actually Work

Good MIG welds on galvanised steel come down to the consumable, the shielding gas and how the machine is trimmed. Get them right and the zinc burns off ahead of the pool, the bead comes out smooth and the spatter stays on the work. Get them wrong and you are back to the popcorn bead from the intro.

Three consumables cover most galvanised jobs:

• ER70S-6 solid copper-coated wire is the default for light-gauge sheet and structural steel: clean feed, minimal spatter and the stable short arc that slow-travel galvanised work demands.
• ER70S-5 carries more silicon and manganese for extra wetting or a hotter, less clean job. Choose it when the weld will be galvanised later, since silicon in the deposit changes how the zinc coating grows over it.
• Self-shielded E71T-8 flux-cored wire wins outdoors, in wind or on dirty work: no gas, far less wind sensitivity than a gas-shielded arc, more penetration on heavy sections, at the cost of slag, spatter and a bead needing grinding.

Shielding gas: pick argon-CO2 over pure CO2

Run a 75/25 or 90/10 argon-CO2 mix. The AGA is explicit: more heat, slower travel and an argon-CO2 shielding gas in GMAW give a more stable arc, smoother deposits, minimum spatter and less zinc loss. Pure CO2 works and melts deeper, but raises spatter on coated stock, and spatter means adhered zinc crust to chisel off later.

• 90/10: more argon, so a more inert, stable arc with less penetration but lower spatter. The friendly choice on thin galvanised sheet and anywhere you want a clean bead over deep fusion.
• 75/25 is the general-purpose pick most shops have on the wall. Set flow for the nozzle and check it with a flowmeter before you start: a lean mixture on a hot job will spit.

Wire diameter and machine setup

• 0.030 in and 0.035 in for thin material and the best arc control. The arc wants to be short and controllable; finer wire gives a smaller contact tip, shorter stickout and tighter arc that pushes the zinc bead forward instead of engulfing it. Go to 0.045 in for thicker plate, typically 3/8 in and up, where deposition rate and penetration pay off.
• 0.030 or 0.035 is a cable-length and duty-cycle decision, not a metallurgical one. The 0.030 runs cooler and loads the gun less, which matters on a long lead or a 30 m feeder; the 0.035 gives more heat per inch of travel and pushes burn-off through a heavier coating. Shops keep the 0.035 for structural work and the 0.030 under 1/4 in.
• Machine capacity: 250 A vs 300 A matters more on galvanised steel than bare steel. Burn-off means holding a stable arc for long stretches without cranking travel speed, and a machine at its output limit hunts, wanders and changes arc length, the difference between a smooth and a wormy bead. A 300 A source on a 250 A machine is a real upgrade. Keep feeder leads short and clean the gun neck: zinc vapour condensing in the cable liner chokes wire feed before the arc.

Trim the machine properly

• Inductance (C25): turning it up softens the arc, cuts spatter and widens the puddle, giving the zinc more time to burn off and a slightly longer arc less likely to blow the coating out of the pool. The factory default of a hard, short arc is fine on clean steel but stiff and spattery on a zinc-coated joint. Nudge it up a few notches and re-test.
• Contact tip to work distance: stickout of roughly 1/4 in to 3/8 in, nozzle about 1/2 in from the work. Short stickout keeps the arc focused and the spatter down; long stickouts scatter the arc, spit zinc droplets and promote crusting.
• Nozzle management: zinc vapour crusts over the gas cup and contact tip no matter how good your settings are, so plan a mid-job change. Keep a spare nozzle, a spare contact tip and a brass wire brush at the bench, and clean the nozzle with the brush. Never put an angle grinder on a nozzle: it puts iron into the arc and into the weld. Grinders belong on the grind-back zone before welding, not on a hot nozzle.

Indicative starting parameters

Starting points only. Verify each one on a test coupon cut from the material and coating in front of you.

Material / condition	Wire	Gas	Wire feed speed	Travel speed	Notes
16 ga (1.6 mm) electro-galvanised sheet	0.030 in ER70S-6	75/25 Ar-CO2	Low, 200-250 in/min	Slow; keep arc short	Burn-through risk; use a backing bar
3/16 in (5 mm) hot-dip plate	0.035 in ER70S-6	75/25 Ar-CO2	Mid, 250-320 in/min	Slow; weave on butts	Expect spatter; anti-spatter spray
1/4 in to 3/8 in structural HDG	0.035 in ER70S-6 or 0.045 in	75/25 or 90/10 Ar-CO2	Mid to high	Slower than bare steel	More amperage to burn the thicker coating
Heavy section, field or windy site	0.045 in E71T-8 flux-cored	None, self-shielded	250-400 in/min, set to suit current	Moderate; do not rush burn-off	DCEN (electrode negative); grind slag off

Adjust from there. A porous, wormy bead wants more heat or slower travel. An undercut, wide bead wants a shorter arc and less amperage. If spatter is climbing the nozzle, slow down, check the gas mix and try a 90/10 blend.

One closing safety note: the AGA points to AWS/ANSI Z49.1, Safety and Cutting in Welding as its reference for welding safety and health. Keep the current edition on the shop wall next to your settings chart.

For the spec-sheet detail, see our guides to choosing MIG wire and filler metals and picking a shielding gas cylinder and mixture, including 75/25 versus 90/10 blends and how to read a wire certificate.

Weld Before or Weld After Galvanising? Repair, Retrofit and Touch-Up

The order of operations is the single most important decision when MIG welding on galvanised steel. If the part can fit inside a hot-dip galvanising kettle, the job should be planned for welding first. If it’s already coated, too large to dip, or needs an on-site splice, you’ll be treating it as a repair operation with its own set of precautions.

Welding First: Set Up For a Full Coating

For new fabrications, welding before galvanising is the preferred route. As the American Galvanizers Association notes on welding before hot-dip galvanising, “unless a welded structure is too large for the galvanizing kettle, best practice is to weld prior to galvanizing to ensure the entire structure is coated with corrosion-inhibiting zinc.” That means every joint, edge and internal surface gets the same protection as the rest of the steel.

That full coverage comes with a requirement though. The galvaniser cannot compensate for contamination left from welding. To ensure a clean bond, there are a few key checks to make:

• Remove flux residue. Welding flux is chemically inert in the galvaniser’s cleaning baths, so it will not dissolve. It must be stripped by wire brushing, chipping, grinding, a pneumatic needle gun, or abrasive blast before the part reaches the kettle.
• Match your consumable. A weld metal chemistry that’s close to the parent steel will promote a more uniform zinc coating across the joint.
• Watch for silicon. Over-siliconised fillers can speed up the zinc-iron reaction inside the kettle. This creates the classic swollen weld bead, which is cosmetic rather than a structural failure, but one worth avoiding where possible.
• Consider heavy weldments. For larger, heavy assemblies, a submerged arc method is recommended as it lays down consistent deposits with less surface residue to clean up.

The biggest benefit beyond coverage is safety. With no zinc present at the time of welding, there’s far less chance of generating zinc vapour, making fume control much simpler for the fabrication shop.

Galvanising First: Repairs, Retrofits and Touch-Up

When welding into an already galvanised structure, the zinc coating becomes a hazard to the weld itself. Before any arc is struck, the coating must be stripped. Per AWS practice referenced by the AGA, including AWS D19.0, Welding Zinc-Coated Steels and AWS D1.1, zinc must be removed from at least 1 to 4 in (2.5 to 10 cm) on both sides of the weld zone, and on both sides of the member.

Once stripped, the weld must be completed to a qualified WPS. From there, the bare strip cannot be left exposed. As laid out in the AGA’s guidance on repairing hot-dip galvanised steel, this area must be restored with zinc-rich touch-up paint or a thermal spray (metalising) system. Skipping this step is what turns a minor repair into a corrosion point. Even with a good restoration, a galvanised-then-welded joint will almost always show a tonal difference from the surrounding coating.

Two specialist applications require extra care:

• Stud welding: Zinc must be removed from both the stud end and the base surface. If left in place, zinc vapourisation can cause violent expulsion of weld metal from the joint.
• Friction welding: Flat-ended studs cannot be friction welded to galvanised plate, as the zinc alloy layers act as a low-friction bearing surface. Pointed studs with a 120 degree point give the best results.

For these repairs, process choice matters. Laser welding systems can help limit heat input on coated sheet, whilst for manual field work, it’s worth weighing up the advantages of stick welding against MIG for the conditions on site.

Factor	Weld, then galvanise	Galvanise, then weld
Coating coverage	Complete across welds, edges and internal surfaces	Local bare steel that must be made good
Corrosion protection	Full zinc coverage for long service life	Dependent on the quality of touch-up
Fume risk	Minimal, as no zinc is present	High, as zinc vapourises; extraction and PPE required
Appearance	May be swollen over silicon-rich welds	Tonal mismatch, even after restoration
Typical use	New frames, gates, handrails, ductwork	Field splices, retrofits, collision repairs

Common Mistakes and How to Fix Them

Zinc is hard to weld through. It boils at a temperature the arc reaches before the steel beneath it melts, so every galvanised weld defect has a traceable cause: too little heat, too much travel speed, or zinc left in the joint.

1. Porosity and the Wormy, Popcorn Bead

The classic defect: a pitted, lumpy bead that looks like popcorn or a worm trail, and it is weak. The cause is gas trapped in the pool, usually zinc vapour the arc could not burn away.

• Travel speed too fast. The pool closes over zinc vapour before it escapes.
• Not enough heat to vaporise the coating. It must burn off ahead of the pool front.
• A dirty or damp joint. Oil, grease, moisture and rust all make gas.
• Wrong polarity on gas-shielded wire. Most MIG guns run DCEP (wire negative), correct for solid wire; electrode positive changes burn and wire melt rates, causing erratic deposit and spatter. Check the gun connections before blaming the coating.

Fixes: slow the travel, grind back a heavily coated joint, clean to bright metal, and confirm smooth wire feed and correct gas flow. The AGA’s guidance on welding galvanised steel notes penetration depth is reduced in both GMAW and SMAW of galvanised steel, so butt joint root openings must be increased over uncoated surfaces.

2. Excessive Spatter

Spatter on galvanised steel is not a mystery. The American Galvanizers Association is direct: spatter formation is directly proportional to zinc coating thickness, so hot-dip steel spatters more than continuously galvanised sheet.

• Pure CO2 shielding. A 75/25 or 90/10 argon-CO2 blend runs a cleaner arc.
• Too short a stickout. Short wire extension, unstable spitting arc.
• Wrong gas mix or low flow. Upsets the arc, oversized droplets.
• Excessively long arc. Droplets oxidise before they land.

Fixes: switch to an argon-CO2 blend, apply spatter release compound so beads do not fuse to the work, fit a correctly sized nozzle, and slow down.

3. Lack of Fusion and Shallow Penetration

Some penetration loss is expected on coated steel, since zinc vapour displaces molten metal at the pool front. A bead sitting on top of the work rather than into it is technique, not coating.

• Increase the root gap on butt joints to compensate for reduced penetration.
• Weave side to side so the pool sweeps clean steel ahead of the puddle.
• Raise amperage or wire feed speed so heat melts base metal, not just boiling zinc.
• Check the C25 (inductive) setting: too high chops the wire tip, too low leaves a spattery stub. Self-shielded wire that is a flux-cored wire behaves differently on DCEN.

4. Undercut and Excessive Reinforcement

Undercut is a groove melted along the toe of the bead that eats into sound base metal. It appears when amperage was cranked up to burn off a thick coating, then compensated for with a long arc. Shorten the arc, set the travel angle, and lead the torch slightly. If the bead is too proud and convex, you are running too much heat for the thickness: back the wire feed down, lengthen the arc slightly, and let the bead fill in.

5. Cracking in the Weld

Zinc that survives in the bead is a serious problem. Zinc is weak and brittle, mixes into the deposit, and can promote cracking along the fusion boundary and in the HAZ. Check for hydrogen on quenched or restrained joints and check fit-up; toe cracks signal residual zinc folded into the bead.

The cure is prevention: exclude the zinc. Grind back before welding, or burn off enough coating that none remains in the puddle. AGA’s guidance recommends removing the zinc coating at least 1 to 4 in from either side of the weld zone. Stud welding carries a similar warning: a zinc coated stud end face can violently expel weld metal as zinc volatilizes from the faying surface.

6. Zinc Crusting the Nozzle and Gas Cup

Not a weld defect, but it will destroy one. Vapourising zinc condenses inside the gas cup and nozzle, building a crust that chokes gas flow, distorts the arc and spatters molten metal onto the tip. Plan a mid-job nozzle change on any long run, then clean the tip and cup with a brass wire brush, never a grinder: grinding leaves ferrous contamination that contaminates the next stainless or aluminium weld.

7. Burn-Through and Distortion on Thin Galvanised Sheet

Thin sheet, typically 18 to 24 gauge, glows through before you finish the pass. The fix is heat management, not speed.

• Back it with a copper or steel sacrificial strip under the joint.
• Lower amperage and wire feed speed for a smaller, more controllable pool.
• Use pulse MIG to avoid cutting through on a short arc.
• Clamp strategically, weld from the outside in, or stitch with tacking spaced out to let the sheet cool.

Remember that zinc fume is a serious exposure, not a nuisance odour. NIOSH notes that welding fume may damage the small airways and cause interstitial pneumonia in addition to metal fume fever, so these fixes matter for your lungs as well as your weld.

MIG vs Stick vs TIG: Choosing the Right Process for the Job

Most fabricators reach for the MIG set out of habit, and on galvanised steel that habit is wrong often enough to matter. Zinc changes process selection less about bead quality and more about heat input, travel speed and fume. The American Galvanizers Association draws the line clearly: GMAW suits thinner material under 1/2 in (13 mm), while steels thicker than 1/2 in are recommended to be welded with SMAW when the coating is left in place. That one sentence is the argument in miniature.

MIG (GMAW): Fast, Clean, Capped Around Half an Inch

GMAW wins on the obvious numbers. Deposition rate is high, the arc is easy to control, and an argon-CO2 blend lays down a smooth, slag-free bead with almost no cleanup. On light-gauge sheet, ductwork, handrail and structural members up to about 1/2 in, that is the right tool. Its weakness is heat management: the arc has to travel slowly enough to vaporise the zinc ahead of the pool front, so the productivity advantage shrinks as section thickness climbs. Wire, gas and machine settings matter here as much as anywhere, and self-shielded flux-cored wire becomes attractive when you want deposit rate in a windy or dirty bay.

Stick (SMAW): Slower, and That Is the Point

On coated steel, stick wins for one reason: time. The slower travel speed gives zinc more opportunity to burn off ahead of the bead, and the flux coating captures vapour and spatter better than a bare wire in a gas stream. The AGA technique is specific: apply the electrode more slowly than normal with a whipping action that moves the electrode slightly forward of the pool and then back into it, so all the zinc is burned off before the bead progresses. Weaving and multiple weld beads should be avoided, and heat input kept to a minimum to avoid undue damage to the adjacent coating. That is a technique, not a machine setting, and it needs practice. Our walkthrough of why SMAW still earns its place covers the fundamentals, and the all-position rod guide helps you pick an electrode for out-of-position work.

The catch is fume. Stick generates more fume per foot of weld than gas-shielded MIG, and on coated steel that fume is largely zinc oxide. Every extra foot adds to what your extraction has to catch, which is why OSHA’s welding fume guidance treats process choice and extraction as one decision rather than two.

TIG: The Cleanest Bead With the Worst Fit

TIG gives the cleanest bead and the tightest control, which sounds ideal until you remember what sits under the arc. Concentrated heat vaporises a wide swathe of coating for very little weld metal deposited, the filler and the tungsten pick up zinc, and a contaminated tungsten has to be dressed or discarded. Most shop guidance says skip TIG on galvanised steel and use MIG or stick instead.

The Specialist Processes the AGA Lists

• Oxyacetylene fusion welding. All grades of hot-dip zinc-coated steels can be welded this way, but low travel speed affects the coating over a much greater area, and welds should not be remelted with a flame because that causes additional loss of zinc coating.
• Stud welding. Fast and tidy on bare steel, but if the stud end face is still zinc coated, weld metal may be violently expelled from the joint because of volatilisation of zinc from the faying surface. Remove the coating first.
• Friction welding. Heat and pressure push the zinc out of the joint mechanically instead of vaporising it, so fume is minimal. It needs specialist equipment and reasonably heavy material.
• Resistance welding. Suitable for material less than 1/4 in thick with a coating lighter than 1 oz/ft², where the zinc is thin enough not to disrupt the joint.

Process	Fume exposure	Spatter	Deposition rate	Positional control	Field suitability	Typical thickness
MIG (GMAW)	Medium to high	Medium on coated stock	High	Good	Good indoors, fair outdoors	Under 1/2 in (13 mm)
Stick (SMAW)	High per foot of weld	Low to medium	Low	Very good	Excellent outdoors	Over 1/2 in (13 mm)
Flux-cored (FCAW)	High	Low to medium	High	Good	Excellent in wind and dust	1/4 in and up
TIG (GTAW)	High and concentrated	Very low	Low	Excellent	Poor outdoors, needs clean gas	Thin gauge
Oxyacetylene fusion	Very high	Moderate	Very low	Limited	Shop use only	Thin sheet

Pick the process that gives zinc the most time to disappear ahead of the pool, then match your extraction to the fume that process produces. Under 1/2 in with light coating and a clean bay, MIG wins on speed and bead quality. Heavier with the coating left in place, the slower whipping action of stick is the AGA’s recommendation, fume penalty priced in. TIG is for the odd repair where the coating has already been removed anyway.

A Step-by-Step Workflow, Real Scenarios and What to Take Away

The arc is the easy part of MIG welding galvanised steel. The order of operations is what separates a clean, compliant job from a porous, fume-heavy mess nobody wants to sign off. Here is the sequence worth printing on the shop wall, followed by four jobs that show how it flexes in the real world.

The shop-wall sequence

1. Identify the coating and estimate its weight. Hot-dip galvanised (thick and spattery), electro-galvanised (light and easy), or continuous galvanised sheet (in between). Coating weight in ounces per square foot tells you what you are signing up for.
2. Decide: grind back or burn off. For load-bearing joints, follow AWS D19.0 and strip the coating within 1 to 4 in (2.5 to 10 cm) of the weld. Burn-off is for field repair, sheet metal and retrofit work.
3. Set up extraction and respiratory protection before striking an arc. Local exhaust at the source, plus a respirator and full PPE. OSHA requires employers to control hazardous fume and gases during welding, using ventilation and, where engineering controls fall short, respiratory protection.
4. Prepare the joint. Grind, wire brush, confirm bright bare metal, then fit up. Butt joints need a larger root gap than on bare steel because penetration drops.
5. Set the machine. ER70S-6 at 0.030 or 0.035 in, a 75/25 argon-CO2 blend, C25 near mid range, and a travel speed slower than you are used to. Verify wire drive and gas flow as part of your power source, torch and gas gear check.
6. Run a test coupon on the actual material and coating. Same thickness, same coating, same position. Cut it open if the joint is critical.
7. Weld with the right geometry. 10 to 15 degree drag angle, short arc, and a side-to-side weave on butt joints to make up for the lost penetration.
8. Manage the nozzle and the spatter. Keep spares close. When zinc crusts the cup, swap the nozzle and clean it with a brass wire brush, never a grinder.
9. Inspect the bead. Porosity, undercut, cold starts and fusion. A bent coupon tells you more than a passing glance.
10. Clean up mechanically. Wire brush, chip, grind or lightly abrasive blast. Per the American Galvanizers Association, all weld flux and spatter must be removed by mechanical methods such as wire brushing, light abrasive blasting or grinding before galvanising.
11. Restore the coating. Where zinc came off, apply zinc-rich paint or thermal spray. Zinc-rich touch-up is a standard part of hot-dip galvanising practice, and repaired areas must be built up to the specified coating thickness.
12. Record your settings. Wire, gas, wire feed speed, amperage, travel speed, C25. The next similar job starts ten minutes ahead.

Four jobs you will actually meet

Farm gate hinge, workshop. Small hot-dip gate, structural enough to matter. Grind back a finger’s width on each side rather than burning through 3/8 in hinge steel, MIG it with 0.030 wire and a 75/25 blend, then touch up with zinc-rich paint.

Galvanised handrail on a mezzanine. Thin tube where appearance counts. Grind back and paint rather than burn off, and treat indoor extraction as the priority, not an afterthought.

Galvanised HVAC ductwork. Light continuous galvanised sheet, seams and spot welds. Low amperage, small wire, a backing bar to stop burn-through, and a travel speed slower than feels right.

Structural tube connection on site. Hot-dip square tube, load-bearing, welded in the field. Full AWS grind-back zone and a qualified procedure, and on thicker wall it may be worth switching process. The case for why SMAW still wins on coated steel applies here, and our all-position rod guide covers the vertical passes. Field fume control is the hardest part of this job.

What to take away

MIG welding galvanised steel is entirely doable. It stops being guesswork once you accept the fact that drives everything: zinc boils away at 907 °C while the steel beneath it is still solid. That single piece of physics sets your heat input, travel speed, root gap, wire and gas choice, and how much spatter you will be cleaning off the bench tomorrow.

Two things are non-negotiable. Fume control: extraction first, respiratory protection and PPE always, and remember the AGA’s plain instruction that galvanised welding belongs in well-ventilated locations. Coating integrity: either remove the zinc cleanly and repair it afterwards with zinc-rich touch-up or thermal spray, or burn it off deliberately under control. Never do neither.

Quick FAQ: Can you MIG weld galvanised steel without removing the coating? Why does it spatter so much? Should you use pure CO2 or an argon blend? Does a ground-back joint need painting afterwards? Short answers to all four are in the American Galvanizers Association’s guide to welding galvanised steel and throughout this article.

Frequently Asked Questions

Can you MIG weld galvanised steel?

Yes. Galvanised steel can be MIG welded routinely, and AWS D19.0 exists specifically as the procedure specification for welding zinc-coated steel. The method is either removing the zinc coating around the joint before welding, or burning the coating off in front of the pool with a slower travel speed and more heat, while controlling the fume.

Do I need to grind the zinc off before MIG welding?

For structural work, yes. The American Galvanizers Association notes that AWS D19.0 calls for welds on areas free of zinc, and that the coating should be removed at least 1 to 4 in (2.5 to 10 cm) from either side of the weld zone and on both sides of the piece. For sheet metal, ductwork and repair work, welding through the coating with a burn-off technique is common practice.

What happens if I weld straight over galvanising?

The zinc boils away long before the steel melts, so the vapour has to go somewhere. It gets trapped in the weld pool, causing porosity and a lumpy, wormy bead, it spits out as heavy spatter, it reduces penetration depth, and it produces a dense zinc oxide fume that is genuinely hazardous to breathe.

Is MIG welding galvanised steel dangerous?

The fume is the main hazard. Zinc oxide fume is regulated by OSHA at a PEL of 5 mg/m3 as an 8-hour TWA, and NIOSH sets the same REL with a short-term limit of 10 mg/m3. Overexposure can cause metal fume fever, and NIOSH has estimated that 30 to 40 percent of welders have experienced it at some point. Local exhaust ventilation and suitable respiratory protection are the controls to rely on.

What shielding gas should I use for galvanised steel?

Use an argon-CO2 blend such as 75/25 or 90/10 rather than pure CO2. The AGA specifically notes that increasing the heat, reducing welding speed and using an argon-CO2 shielding gas gives a more stable arc and produces smoother weld deposits with minimum spatter and zinc loss, while spatter increases noticeably when welding galvanised steel using CO2 shielding gas alone.

Why is my galvanised steel weld full of holes?

It is almost always trapped zinc vapour. Travel too fast and the zinc ahead of the pool does not have time to burn off, so the gas is captured in the bead. Slow down, add a little heat, open the root gap, and if the porosity persists, grind back the coating around the joint and weld clean steel instead.

Can I weld galvanised handrail and ductwork without grinding it back?

For light-gauge sheet, ductwork and handrail, yes, many shops weld straight through with a burn-off technique and local fume extraction, then treat the joint. Structural and pressure-retaining connections follow the AWS rules instead, which means removing the coating around the weld and restoring it afterwards with zinc-rich paint or thermal spray.

What wire and settings should I start with?

A copper-coated solid wire such as ER70S-6 at 0.030 or 0.035 in with a 75/25 argon-CO2 blend is the most forgiving starting point for thin to medium material. Use 0.045 in for thicker plate, and consider a self-shielded flux-cored wire such as an E71T-8 class for outdoor or dirty conditions. Always run a test coupon on the actual material and coating before committing to production work.