[Davidf]

My suggestion, mostly like others have already said: Prime with epoxy initially. Then within 24 hours, apply a coat of lacquer primer. The lacquer primer will protect the epoxy primer from UV degradation. Sure, the lacquer primer will absorb water, but the epoxy primer will keep it away from the metal. When you are ready to paint, the lacquer primer can be easily removed via sanding and the process will provide the necessary tooth to the epoxy primer for recoating as needed.