I usually wire brush and/or wire wheel the rust off; then I hit it with rustoleum rust converter. Let that do it's thing for at least 24 hours; then cover with rustoleum satin black, followed by some satin clear. No primer needed, the converter creates a good surface for the paint to adhere to - though I suppose if you wanted to prime, I would go with a bonding primer.
I typically do this in later fall, before winter (just clean up all the rock scrapes and what not); and then check again in spring after winter breaks (stupid road salt!).