Rob, I have had good luck with Rustoleum "Hammerite" type paint. The main reason I have stayed with it is not because of superior rust proofing (none of the spray can stuff is), but because these projects always get torn down, modified, repaired, derusted, etc. This paint removes pretty easily with plain old lacquer thinner, even after a few years. It has ground glass in it, which seems to let the thinner cut through easier. I do NOT put primer under it, and you can respray areas you might rework without having it look too bad. Also, you cannot spray lacquer paint over it, it wrinkles.
When the rust comes, and it does, its easier to deal with if you dont have to grind or sand or burn off coatings. This is really a problem in the deep spots of weld corners, where you have to get all the paint off and use something like OSPHO to convert the thin rust lines that sometimes form.
Its not supposed to work, but the rails on my flatbed sides (1" tubing) get rusty from scuffing and work. I have been putting the OSPHO right onto the rusty paint areas, keep agitating it to keep it wet, and then flush with the garden hose. It turns the rust gray in my gray Hammerite paint! Eventually, after enough treatments over time, the steel turns that old "purple iron" shade and doesnt rust anymore.
It does tend to melt the glass if you start grinding on it without first using the lacquer thinner or acetone.
Regards, JimL