For me, that would probably be a surface-tolerant epoxy, like Devoe bar-rust 235. For a steel hull, I would prefer some anti corrosive properties in that epoxy.
However, for exterior use, these wonderful epoxies will need top-coating, so maybe you would disqualify them?
So, my answer is that I need two paints.
2. Two part linear polyurethane topcoat.
This system works on wood, fibreglass, steel and concrete.