For surface rust the best answer is to use a phosphoric metal prep before painting.
Rust is like cancer. Unless you get it all, it will come back and spread.
By using something like Ospho or Kleenstrip's 'concrete and metal prep' you convert iron oxide to iron phosphate (which is more or less inert)
The thin liquid will be drawn into pinch welds and flanged edges fixing the problem where it starts.
If panels have deep pitting or rust through, you really need to cut it all out and weld in solid metal.
If you stay tuned i'll show how bad it can get.
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.