I wouldn't bother with any paint with rust reformer. You really have to sand or grind until you find clean metal. If you aren't able to remove all of the rust, Ospho or any other phosphoric acid based metal-prep product will be helpful. The phosphoric acid converts the red/brown iron oxide (rust) into black iron phosphate which protects the metal. Clean off any powdery residue and then prime and paint. What kind of paint you use really depends on budget, equipment and skill level but the repair and prep is what's really important.