Butt Welding 18 Gage w/MIG?

I'll throw in two cents worth here with respect to the original post. I like to use the copper backer trick, especially on thin metal like body panels. It provides an instantaneous heat sink and the weld metal will not stick to it. If you are trying to bridge a gap or fill a hole you can run the bead right across the copper and not burn holes. In theory you could weld a 1/2" hole shut with a backer behind it.

I have several sizes of bar that I have cut and bent into convenient shaped pieces for backing. If you follow the good advice above and try the copper backer as well you will see good results.