[Sirrotica]

I learned many years ago when a panel is brazed to another piece of metal that you absolutely have to get rid of the glassy looking residue that comes from the melting of the flux used on the brazing rod.

About 50 years ago, before there were MIG welders in every body shop, body men that didn't know how to oxy acetylene weld with steel rod, would use the brazing process, similar to soldering with brass. Brazing was easier to learn than the welding process, so many times it was used as a substitute for welding when replacing body panels. If you wanted anything to stick to the brazed joint, you have to remove the flux residue, either by sand blasting, or grinding.

It's very possible the flux residue was never removed, I know for a fact that no filler, or primer will bond well to the flux residue. I used to work for a guy that did body work as the bottom guy that did all the sanding, and crappy work. He sand blasted the brazed area after he was done, before he applied any filler so it would get a good bond, and remove all the flux residue.