I'm going to paint the surface of a helipad 30 miles off shore. It hasn't been painted in over 20 years and has a variety of coatings on it. Places are worn down to the bare steel, most has a layer of worn out latex with red primer showing through. Material cost is crucial, performance is next important. I'll power wash first, but since it's a mix of bare metal and latex under a subtropical sun, was considering either an oil based primer (likely blo/naptha) or a pigmented shellac. 6800sq feet. Suggestions?