MIG Welding Sheet Metal
Article by Mark Trotta
When repairing rusted body panels, the more time that you spend on the metal work, the less sanding and filler will be needed to finish the repair.
MIG welding is the preferred method for most automotive bodywork, and since sheet metal is the thinnest of metals, a large, expensive MIG unit is usually not required.
The majority of sheet metal welding that I've done was with a 90-amp, 110-volt welder. There's nothing wrong with using a larger machine, it's just not necessary when welding thin metal.
Butt Weld vs Lap Weld
Lap welds are used when quick results are needed. This is why production body shops swear by them. A lap weld is much easier than a butt weld, but doesn't seal correctly, and will promote moisture from the back side. An overlapping joint will provide a place for moisture to collect and rust to take over again.
Because cars are running machines, vibration causes body panels to flex. Lap welds, because they cannot flex like the sheet metal around them, will eventually show visible creases.
Butt-Welding Sheet Metal
The best way to weld sheet metal panels together is by butt-welding, which is joining them side-by-side (see picture below). Butt-welded panels withstand stress better than any other type of weld joint.
To obtain the best results while MIG welding sheet metal, proper fitment and cleanliness of the metal are essential. First, make a patch panel and then cut off the old sheet metal.
Spend the time to make your patch a perfect fit. Ideally, there should be a 1/32" gap between the patch and the body panel.
Zinc Primer Spray
For extra protection against future rust, spray the back side of the panel with a zinc-rich primer. These are quick drying primers and offered by many companies, including U-Pol.
The zinc primer gets sprayed on after trimming but before welding.
Hold The Panels In Place
After cleaning the bare metal, clamp the panel securely in place. There's dozens of methods for doing this. Vise-Grips or specialty welding clamps can be used to hold it in position for welding. The ones in the pictures below are butt welding clamps.
Shop: Butt Welding Clamps (16 pieces)
When MIG welding a patch panel onto a vehicle, you need to leave a small gap between the patch panel and the original metal. The gap is to compensate for expansion when the metal gets hot. Test fit the panel until an even 1/8" gap is left around the patch. This small gap will be filled in by the weld puddle.
After fitting the patch, clean the panel with acetone to remove any grease or residue for a clean weld seam. Now its ready to be tack-welded into place.
Tack-Weld Patch Panel In Place
Thin gauge sheet metals are more difficult to fit up for welding than most other metals. Tack welding is used as a method of holding the panels together during the welding process without warping them from the heat. Tacks about one inch apart usually suffice.
Place the welding tip wire at the gap and tack-weld several small spots, spacing them evenly apart. Tack in a few places, apart from each other, and wait until the panel is cool to the touch. Then tack between the two previous tacks.
Avoid Warping the Sheet Metal
It's very possible to warp the metal on your car while welding the patch panel in place. Begin the final welding with one small portion at a time.
CAUTION: Do not try to run a weld bead from one end to the other!
Weld about an inch of bead at a time. Move from the top to the bottom. Start with a weld on the top of the patch, and let it cool (a little compressed air helps). Then tack weld on the bottom and let it cool off. Use the same technique for the sides, until you have one solid bead all the way around the patch panel.
Read: MIG Welding Helmet Review
Some bodymen prefer to pause after each short weld bead, then hammer the weld (if you can get a dolly behind the patch). This helps "relax" the panel back to its natural shape.
NOTE: The above procedure is to done with a body hammer and dolly.
TIP: If you can get behind the weld, use a copper backup block as a heat sink while welding. This helps minimize warpage. Again, wait until the panel is cool to the touch. Correct any misalignment before you go any further.
Allow the metal to cool (cool to the touch) before adding more welds. Eventually, all of your short seam welds become connected and you have a fully welded panel. When a patch panel is welded in correctly, it leaves a weld joint that's nearly invisible and will need very little filler.
Problems With Welding Sheet Metal
If you are burning through the metal, you're not moving the torch fast enough and/or your welder is set too high. Move the torch faster, and/or reduce the amp setting of the welder and the lower the wire speed. Using a thinner wire (example; 0.30 instead of 0.35) will also help.
Once the welding is done, grind the welds down in preparation for final finishing and primer. A 36-grit disc on an air grinder will easily level the high spots, or you can use an angle grinder with a flap disc.
Don't spend too much time grinding on one spot, as that could heat and warp the metal. Any low spots will require body filler.
If you live in a relatively high humidity area, coat the patch panels as soon as you can. Surface rust will form almost immediately if you don't. Spray patches with a coat of self-etching primer (a spray can is fine for this).
It's All About Practice
Once properly set up, MIG welding is clean and fast, and works well with thin metals. It is the easiest welding process to learn, and most commonly used for automotive body work. I have done a lot of patch and panel welding, and through trial and error, I'm always learning better techniques. Like any skill, it's all about practice, and there's really no limit to how good you can weld.
Welding helmets protect your face from sparks and your eyes from harmful rays. The best choice is an auto-darkening helmet.