There's some stuff called "Rust Doctor" that will stop the rust and then you can paint over it.
http://www.therustdoctor.com/ I used another brand that worked well, but I can't remember the name of it. You have to get the worst part (read loose) of the rust off and then you put the rust killer on. It will turn black and when it dries you paint right over it. My frame looks new now. I used "Rustoleum" paint over the rust killer.