Salt air eats a standard paint job in two years. Here is what to specify instead.
For inland service, a two-coat epoxy primer and polyurethane topcoat is enough. For coastal or island service, add a zinc-rich primer and increase the dry film thickness to 240 µm.
Blast to Sa 2.5 before painting. A paint system applied over mill scale will delaminate at the scale boundary, and that is where rust starts.
Specify stainless fasteners on coastal units. Painted mild steel bolts are the first thing to show rust, and they are the hardest to replace later.