MIG Welding Sheet Metal
While 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 of most automotive bodymen, and since sheet metal is the thinnest of metals, a large, expensive MIG unit is usually not required. The majority of the sheet metal welding I've done was with a 90-amp, 110-volt welder.
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. Another point to consider is cars are running machines (hopefully), and vibration causes the body 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.
read Making A Patch Panel
Take some time to make your patch a perfect fit. Ideally, there should be a 1/32" gap between the patch and the body panel. To obtain the best results while MIG welding sheet metal, proper fitment and cleanliness of the metal are essential. After cutting and fitting the new panel, you are now ready to weld the patch panel in place. The backside of the patch should be sprayed with etching to prevent rusting.
After cleaning the bare metal, clamp it 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 clamps in the picture above are called "Klecos". The ones in the bottom picture are called "Interlocks".
When MIG welding a patch panel onto your car, you need to leave a small gap between the patch panel and the original metal. Test fit the panel until only an even 1/16" gap is left around the patch. This small gap will be filled in by the weld puddle.
Tack-Weld Patch Panel In Place
Thinner-gauge metals are more difficult to fit up for welding. Tack welding is used as a method of holding the components during the welding process. Tacks about one inch apart usually suffice.
Carefully fit the patch, then go over the entire panel with acetone to remove any grease or residue for a clean weld seam. Now its ready to be tack-welded into place. 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
While replacing body panels, overheating and warping thin sheet metal is common. Do not try to run a bead from one end to the other! Some bodymen prefer to pause after each short weld bead, then hammer the weld (if you can get a dolly behind the patch) to help "relax" the panel back to its natural shape.
Another tip is to use a copper backup block as a heat sink when welding. This will help minimize warpage. Again, wait until the panel is cool to the touch. Correct any misalignment before you go any further.
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. 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.
Weld about an inch of bead at a time. Start with a weld on the top of the patch, and let it cool (a little compressed air helps). Then 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.
Begin the final welding with one small portion at a time. To avoid warping the sheet metal, 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 (0.30 instead of 0.35) will also help.
Once the welding is done, grind the welds down in preparation for primer. A 36-grit disc on an air grinder will easily level the high spots. Don't spend too much time on one spot, it could heat and warp the metal. The low spots will require body filler.
If you live in a relatively high humidity area, coat the patch panels quickly. Surface rust will form almost immediately if you don't. Spray patches with a coat of self-etching primer.