See if you can find a chromate self-etching primer.
Lots of different regulations in the past few years have made the excellent paint formulations that really worked, alas, illegal.
if self etch is not available, wash with dilute muriatic solution, then wash clear with hot water, and apply a primer immediately after drying.
top coat with any paint that is compatible with your primer, for maximum weather resistance, a two-part epoxy paint will outlast most anything else.