Decades ago, on the alloy body cars-original AC Shelby Cobras, Alloy bodied Ferraris etc, we used fisrt a DuPont Metal Prep, then the DuPont Vari Prime self etching primer. We ended up removing much of the primer coat, since we could not put plastic filler over it, and these bodies required typically wiping the whole car.
This was after we carefully stipped with chemical stripper, then buzzed clean with air da and 120 grit paper( carefully)
They were really rough-the opposite was a Merecedes, where for whatever reason the steel panels seemed resistant to dings etc. Very little filler required on most of those. Don't even ask about a Corvette, but at least glass does not dent.
