With a light coat of self etching primer, you can weld over it with only minimal clean up prior to welding with a wire bruch and some lacquer thinner. IMO it's easier to clean a few spots before you weld then have the entire frame get a bunch of surface rust.

Coating it with oil also works, but is a real pain to get all the oil off of the frame and out of all the little nooks and crannies before you paint it.

Epoxy primer will work, but my paint rep has always recomended self etching primer on bare metal prior to the epoxy primer.

I'm certainly not an expert on it, I rely on what the paint company reps recomend........