People will cut me down but....
Coldgal and salt water are very average combination. You don't see bridges and steel boat stuff painted in coldgal.Perhaps a steel boat forum? Follow the instructions on painting duragal as I had some issues with white powder "rust"corrosion as I didn't do proper prep on a shed frame and my paint result was crap. simple wipe with metho to remove the "factory wax" would have saved me a later touch up. Speak with someone who does industrial coatings as there are better paints than cold gal. As for oil filling or partial oil fill it still never sloshes on the top inside of box so it is fill it up. Why not use a industrial protectant on the inside and spread it by turning over the frame a few times so salt can never tough the metal.
Pulling my head in now. Hard to specify a product as the ones I have seen used recently haven't had years of wear and tear to say to use them. We have painted part our davits ( constant salt spray) with the POR alkeylyde type product and wrapped in some mastic tape stuff products under a maintenance contract and looks ok for now. Like everything in 6 months if it falls to bits I'll wonder if it was worth the money etc etc.