If you or anyone else has sheet metal holes that require a backer, copper is what I’ve used in the past and is the choice material for giving the backside something that the weld won’t stick to.I used the welder to build up material around the holes, quick taps on the trigger, then used the grinder to make them flush filling in pin holes if needed. The next step would have been sanding, bondo, paint, but I skipped the bondo since they will be hidden anyways and it was more of a thing to do.