The worst part of cleaning and rust repair on my fenders was the "tar" like stuff that the factory used to undercoat them. That was after 60 plus years of hanging out and, it worked. No rust under that stuff. It had fallen off in places and rust was there. The modern stuff that is from the same manufacture as the paint you use is what I would recommend. The factory had some under there so I would think a judge would have to expect it....but then maybe not. I know some ACMA judges and they are a fussy bunch.
