There's lots of theories on Rust prevention but I always like to use some sort of rust converter either ospho which is phosphoric acid and you can get a cheaper brand of it at Home Depot in the outdoor household cleanup products or at least Permatex Extend spray on to chemically treat the remaining rust before I coat it with anything. Then I also like to use Rust-Oleum instead of regular paint since it seems to have so much more rust prevention than plain Automotive Dupli-Color paint. After that dries you may or may not want to undercoat it as some people claim the undercoating can hold moisture in and cause rust which might be true but when rust has already started I find the undercoating gives you more time then leaving it exposed.
