I will add that I do similiar to what Denny does. I prime most of the sheet metal parts with epoxy primer, scuff it and then apply the Rustoleum. I tend to not prime cast iron parts and interior parts. Since most everything I paint gets media blasted, adhesion is usually not a problem on non-primed parts.
Wheel side of fender liners get epoxy primer, rustoleum, and then finally undercoating after final assembly (to seal/protect fasteners, etc). I suppose the rustoleum step could be eliminated, but it just looks better while the part is sitting around waiting to be installed.
