Most automotive lines have a "chassis black" or "rally black" that are single stage urethanes, extremely durable, and give a semi gloss finish. You can get custom colors mixed in an appropriate single stage too.
Have your frame sandblasted, prime with an epoxy primer, and paint.
I have not used POR, but understand it is great on a rusty piece, so that is an option if you can't chemically(dip) or mechanically(media blast) strip the chassis first.
I just did mine with Sikkens Epoxy Primer and Rallye Black.
Looks great!
