I don't know what type of welds you are doing except that you are using a MIG welder. A typical weld is a seam weld which generates a lot of heat which can melt the metals if they are thin and warp the panels. Another weld that can be done with a MIG welder is a spot weld. Drill small holes of around 1/8" in the patch near the edge say about every 1" or so. Then fix the panel in place and start welding through the holes to the panel you are repairing. Many MIG welder manufacturers even make special tips with little legs to hold the tip off of the metal with the correct clearance, and because you are not welding at the edge of the patch piece, the patch acts as more of a heat sink and tends to not melt away as much as seam welding on the edge of the patch.

We used to not wear a mask when we were doing these welds, just line up the tip and look away and hit the trigger for a second. The Hobart MIG welder I used had a "spot weld timer" that could be set for a second or so and then it would stop. Use the thinnest wire that will fit in your welder and practice to see how low you can set the welder and still have it flash off well enough so that it is melting metal the second the wire hits, but doesn't burn through the metal. It takes a bit of practice, but it is not rocket science. If you are a crafty guy, and have a decent welder, and you are not trying to weld to rust, you will make it work. Be sure to use seam sealer to encapsulate the bare metal so it does not rust out again.

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Doug

1987 300TD x 3
2005 E320CDI