Go to the auto paint store and get some etching 2 ppart primer.
Wirebrush the door to get as much of the scale and oxide off as you
can. Shoot it with the primer, then shoot on some polyurethane
fortified enamel on it.
That's what I did with an aluminum boat and salt water won't hurt it.

That's a good idea. Just be sure to wear a cartridge mask when you
shoot the polyurethane!