if your just doing lil bits hear and there u can just grab a good spray can primer and give it at least 2 coats, wait for it to dry in between coats. if your unsure if a primer is any good get some steel panel/bit , remove paint to bare metal, paint a bit in primer and leave the rest bare. then you just make it rust by leaving it outside and putting water on it now an then. once you see surface rust get paint stripper out and clean the primer off so you can see if it works.
an epoxy primer is the best ( rubber based), go by what spray painters recommend.