I know a guy who paints for a living. I asked him about using Rustoleum on the under coat to seal the metal. He told me he had a bad experience painting over it one time where the overcoat/finish coat wouldn't stick. He's not sure why, said it probably was a brand incompatibility issue. But since he'll probably spray the truck for me, he said to stay away from it. This is just one guy's opinion. He also did a motorcycle for me, that I swear I primed with rustoleum so who know, just thought I'd throw that out there. The motorcyle turned out great!