Before you choose your frame paint, I'd personally suggest delousing the metal using some kind of media blast. I sort of lean towards Soda due to it being eco-friendly to some degree but whatever is available. Then go nuts with a compressor driven crevice tool and clean out the crap in the frame rails that always escapes the blast. 3M makes a frame coating specially for the inside of the tubes/rails and it comes with a trio of tubes of various lengths to get deep into the frame innards.
After that, well, lots of suggestions have been posted on this board. One that you might want to check out is Eastwood. They offer a frame coating system, or did, my catalogue is several minutes old.
Valspar also offers products and Rustoleum was just mentioned above. Gotta be others 'cause most everyone has their personal favorite.
Somewhere, sometime back, i did post on POR-15. it is not a rust killer, it is a rust encapsulator. The underlying concept is that without oxygen the rust can't propagate. If you use POR be aware that it is UV degradable and has to be top coated or covered to keep it from being injured. It can also abraid or be worn away if on the bottom of a frame rail that sees a lot of gravel or sand based roads.
Nick