I just came in from working on my truck doors. After removing
the brackets that previously held the old style west coast mirrors,| was left with 10 1/4" dia. holes. I went to a local scrap metal dealer and obtained a 3ft pcs of copper flat bar (the type that is used as a bussbar in a electrical panel) 1/4"thick x1" wide I cut several pcs 4" long and used an end mill cutter in my drill press (looks like a router bit and is used in the machinist trade) & cut flat bottom hole and epoxied 2 rare earth magnets into these holes. 1 on ea end. I am using a Mig Wire Feed .023 wire. I lined up the the copper back up underneath the holes & held in place by the magnets imbedded into each end. The results were very good. I was very impressed. In very little time I had all the holes filled. No clamps required. On the bottom panels I cut out the rust made new patches. I installed a pcs of backup metal behing the patch. Used small sheet metal screws to hold the metal, which were removed as I spot welded along I am pleased to say that the repairs are comming along good. I was also having problems with thin metal burning out. This is info that I received from some of the old timers in my car club. I hope this info can help you as much as its helped me.

Good luck

1951 Canadian Chev 1/2-ton Shortbox