Here's a trick that has worked well for me:

Locate a few pieces of copper bar (surplus store or flattened copper pipe) and make some weld backers. I have some pieces of 1/8" and 3/16" bar that I found locally and I cut some pieces from those bars to shape as needed. A couple of spoon shapes work well for filling holes. Attach a piece to a magnet so that you don't have to hang on to it while you weld.

When you weld the sheet metal have the copper behind and pressed against the weld area. It acts as a heat sink and the weld will not stick to it. When filling holes you can almost "weld to air". It's really amazing how well this works!