For galvanized or tin, use an etching primer, or if you're cheap- buy some bulk ospho and dilute it, use a roller and roll it over the metal.
That's what I did.
After that, I searched and found some Dupont and Sherwin Williams Marine grade paint that was surplused out. It's dark grey color, but I didn't care about that. I then applied it over that. 4+ years later, no problem.