For small areas, I have had success with the rust converter, sold at Walmart for around $6 in the auto paint section. It chemically converts the rust, and then you use model enamel or whatever to top coat it.

For large areas of rust, I use the oxalic acid bath (do a search there are a lot of threads on rust and oxalic). I then use the model enamel to touch up. The other huge advantage of oxalic is that it removes the rust inside the frame. Try doing that!