I believe epoxy primer is the only primer that is moisture proof. It’s great if you and not going to finish the project right away.
PPG says it requires a clean metal scratched surface using about an 80 grit sandpaper.
Everything else goes on top including bondo. The idea being to get everything that draws moisture away from the metal and the epoxy as close to the metal as you can.
I know, not everybody does that though, especially the bondo .....
Mike
