Old Tyme Body Work

All Gas weld need flux. The flux on the back of the panel will cause the panel to rust pretty quick unless coated from the back. One reason you always see old body work bubble. Also the first coat of filler on seams should always be a waterproof fiber strand. Very important to seal the seam. Bondo / plastic won't do that. That is why all seams on cars they used lead unless under a vinyl top. Vinyl top cars are always rusted at the seams and below when the top is shot. the filler they used was cheaper then lead.