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Depends on how long you want it to last but the best way is to sandblast the frame then self etching primer with a good grade of topcoat.
It can also be cleaned as best as possible by wire brushing/scraping then using rust proofing paint such as Zero Rust.
It`s too hard and time consuming to thorougly clean a frame without sandblasting but if it`s not an option then hand cleaning will have to do.
In addition to what Fom said, wire brushing the frame with a grinder does a good job if sandblasting is not affordable.
However, coat it with Por15 and it will last forever.. I think Por15 can even be painted over rust.
Walkers sell POR 15, Auto Body Specialties sell Zero Rust.
Bolt Spupply House among others have some form of a rust proof coating, POR 15 seems too expensive to me.
ive heard real good things on this site about Por15.
as far as sandblasting, if it is at all possible do it. we rebuilt a trailer a few years ago, dad cheaped out, didnt sandblast it, bought good paint thugh, and just about the entire trailer has peeled. when we redid the Beater, sandblasted the frame, used tremclad, and the only spots that are showing are where the paint didnt quite get the third coat.
As with almost on any subject there has been pros and cons with POR 15. Have a look in Paint and Bodywork forum.
That`s possibly where I saw the POR15 dicussion, at any rate in there or doing a search will be a good place to start.
thats where i had seen the discussion, paint and body, i thought the major negative is just that it is so thick you really had to watch were you applied it so as to save labour intensive scraping later on