Also take a look at POR-15. It is extremely popular with folks restoring old hot rods and cars. It is designed to be applied over rust, and seals the area, preventing rust from spreading any farther. After having several U-Haul It trailer hitches rust badly on several trucks, I used POR-15 on the new hitch I put on my 2006 GMC PU prior to installing it. 13 years later, the trucks frame and some of the body were shot to rust - the hitch still looked like new. I should have painted the trucks frame with it as well..
Larry