To MIG weld sheet metal: use the thinnest wire your machine will feed, an argon-rich gas, and the lowest settings that still give a stable arc. Then stop welding before the panel gets hot. Almost every hole blown through thin steel is caused by heat that accumulated over several seconds, not by one setting being too high.
Why sheet metal is a different job
On plate, the metal around the weld acts as a heat sink and carries away what the arc delivers. On a 1 mm panel there is nothing to carry it: the heat stays in the few centimetres you are working on, the steel reaches melting point behind the arc as well as under it, and the puddle falls through. This is why the technique below is mostly about time rather than amperage, and why the normal MIG setup needs adjusting rather than just turning down.
The second problem is distortion. Steel expands as it heats and contracts as it cools, and on thin material that contraction pulls the panel out of shape permanently. A door skin welded in one continuous pass will not be flat again. Below about 0.8 mm, MIG stops being the sensible choice at all and TIG takes over, because it separates the heat from the filler - the difference between the two matters most exactly here.
Wire, gas and transfer mode
TWI give the usual MIG wire range as 0.6 to 1.6 mm. Sheet work lives at the bottom of it: 0.6 mm, sold in the United States as 0.023 inch. Thinner wire needs less current to melt, and less current is the whole objective. ER70S-6 is the normal alloy - what the letters and numbers mean is worth knowing before you buy a spool.
For gas, TWI list the options for steel as carbon dioxide, argon with 2 to 5 per cent oxygen, and argon with 5 to 25 per cent CO2. Sheet work wants the argon-rich end of that list. Straight CO2 gives a hotter, deeper, more violent arc, which is useful on plate and exactly wrong here - which gas for which job goes through the trade-offs.
The mode you want is dip transfer, also called short-circuiting. TWI describe it as the molten metal on the tip of the wire transferring by dipping into the weld pool, at low voltage - they give 17 to 22 volts at 100 to 200 amps for 1.2 mm wire. On 0.6 mm wire the numbers are lower still, and the chart inside your machine's door is a better guide than any article, because it is specific to that machine.
You can hear it. Dip transfer on thin steel sounds like frying bacon - a fast, even crackle. A hiss means the voltage has climbed into spray transfer, which will not work on sheet. A popping, stuttering sound means the wire speed and voltage are fighting.
Stitch welding: the technique that actually solves it
Do not run a continuous bead. Weld in short bursts with gaps between them, and let the metal cool in between. This is usually explained as saving the machine's duty cycle, which is wrong - it is the panel's heat you are managing, not the welder's.
- Tack the joint every 25 to 40 mm along its length. Work from the middle outwards, alternating sides, rather than along the seam in order
- Let the tacks cool until you can hold your hand near the panel
- Fill between the tacks with bursts of about one second, each burst overlapping the tack before it
- Move around the joint rather than along it. The next burst should be somewhere the panel is still cool
- If you can smell hot paint or see the steel discolour in a widening ring, stop and wait. That ring is the distortion happening
- Resist the urge to grind each burst flat as you go - grinding adds heat too, and it is quicker to grind the whole seam once at the end
Backing bars, and why copper
A copper bar clamped behind the joint gives the heat somewhere to go and gives the puddle something to sit against instead of falling through. Copper because the weld will not stick to it - steel does not wet copper - so it comes away cleanly afterwards. Aluminium works at a pinch. A steel bar will weld itself to your panel.
Where a backing bar will not fit, the answer is usually fit-up rather than skill. Two edges touching along their whole length can be welded; the same two edges with a one-millimetre gap cannot, because there is nothing under the arc to melt. Time spent clamping and trimming is time you do not spend filling holes.
The shade number for MIG
Lower than most people set, and different from the stick and TIG bands. OSHA publish these for gas metal arc and flux-cored welding specifically.
Filter shade for MIG (GMAW) and flux-cored
| Arc current | OSHA minimum | ANSI and AWS suggested |
|---|---|---|
| Under 60 A | 7 | not listed |
| 60-160 A | 10 | 11 |
| More than 160-250 A | 10 | 12 |
| More than 250-500 A | 10 | 14 |
Most sheet metal work sits in the 60 to 160 amp band, so shade 11. The other half of the problem is what you are wearing: sheet work is done leaning over a panel with the arc close to your forearms, and what a flame-resistant shirt actually certifies is not what most of the labels appear to say.
Reading the panel: what went wrong
Sheet metal MIG faults
| What you see | Why | Fix |
|---|---|---|
| A hole where the weld was | Heat accumulation, not amperage. Or a gap under the arc | Shorter bursts, move around the panel, close the gap or add a backing bar |
| The panel has warped and will not sit flat | Continuous welding, or all the welding done in one area | Stitch, alternate sides, work from the middle outwards |
| The bead sits on top and has not fused | Voltage too low, or travelling too fast for the setting | Raise the voltage a little, or slow down. Check the earth clamp |
| Spatter everywhere | Straight CO2, voltage too high, or a dirty contact tip | Switch to an argon mix, drop the voltage, change the tip |
| Porosity in the bead | Draught blowing the shield away, or a dirty panel | Close the door. Clean to bright metal; MIG will not weld through underseal or filler |
| The wire stubs into the plate and pushes the gun back | Wire speed too high for the voltage | Reduce the wire speed, or raise the voltage slightly |
Frequently asked questions
What wire size should I use for sheet metal?
0.6 mm, sold as 0.023 inch in the United States. TWI give the general MIG wire range as 0.6 to 1.6 mm, and sheet work is at the bottom of it because thinner wire needs less current to melt. Some machines will not feed 0.6 mm reliably, in which case 0.8 mm with shorter bursts is the compromise.
What gas is best for welding thin sheet?
An argon-rich mix rather than straight CO2. TWI list CO2, argon with 2 to 5 per cent oxygen, and argon with 5 to 25 per cent CO2 as the options for steel; the argon-heavy end runs cooler and with less spatter, which is what thin material needs. CO2 gives a hotter, deeper arc that is useful on plate and counterproductive here.
How do I stop blowing holes in thin metal?
Weld less at a time. Short bursts of about a second, moving around the panel rather than along the seam, with time to cool between them. Most burn-through is accumulated heat rather than one setting being too high - and if there is a visible gap under the arc, no setting will fix it.
Can you MIG weld car body panels?
Yes, and it is the standard method in body shops. Use 0.6 mm wire, an argon mix, stitch welding, and a backing bar behind the joint where one fits. Clean the panel back to bare steel on both sides - MIG will not weld through underseal, filler or primer.
Why does my thin steel warp even when I do not burn through?
Because steel expands as it heats and contracts as it cools, and on thin material that contraction pulls the panel permanently out of shape. The cure is the same as for burn-through: less heat in any one place, spread out over time, working from the middle of the joint outwards.
Sources
- TWI - MIG welding: transfer modes and their voltages, wire diameters, and shielding gases for steel
- TWI - What is gas metal arc welding (MIG / MAG)
- OSHA - Eye protection against radiant energy during welding: the GMAW shade bands (PDF)
- ANSI/AWS Z49.1:2021, Safety in Welding, Cutting and Allied Processes (free PDF)
- Lakeside Autobody - preventing burn-through on thin sheet
In short
Thin wire, argon-rich gas, dip transfer, and then stop thinking about settings and start thinking about time. Tack every 25 to 40 mm, fill between the tacks in one-second bursts, move around the panel rather than along it, and put a copper bar behind the joint wherever one will fit. The difference between a welder who can do sheet metal and one who cannot is almost entirely patience.