SBC, As far as I know, all metal preps include phosphoric acid, as that is what converts rust from iron oxide, to iron phosphate. They also have a residual coating, which will protect the metal temporarily, until you can get some primer on it.

I can't tell you much about epoxy primers. I don't use them because I've used urethane primer for the last 15 years. It's never failed on hundreds of jobs, so I'm not about to change.





Deferr, In the "olden days" bondo had a bad rep. Some of this resulted from people using it over paint, and having it fail. I use it over heavily ground metal only, go right to primer, and touch up with two-part glaze. The result is zero failures.

Today there may be resins used in fillers that bond suitably to sanded paint, but you would have to consult the manufacturers before I would recommend taking a chance on it.