You did not mention which island; there are a variety of exposures depending on location; however, …
Good, hot-dipped galvanized steel should last indefinitely, the zinc eventually ozidizing to a protective coating. In such a case, painting would be cosmetic. In severe exposures with prevalent winds blowing inland from over salt water, galvanizing probably works almost as well as Type 316 stainless steel (some may disagree with me). Go to a seaside location and look at the metal street light poles. Unless they are decorative (real or faux cast iron with a paint finish). they are probably galvanized steel. No one does any maintenance to these suckers, and they will last 10 to 20 years unless the exposure is extremely severe. I have recently finished specifying an industral building located in a port area just off Galveston Bay. The structural components are standard metal building system components and the finish is hot-dipped galvanized steel. I have seen other instances of similar applications in this area.
There are some caveats you should remind yourself of.
Caveat No. 1: The zinc coating can be damanged during installation through normal abrasion and welding. While very minor damage to the zinc coating may “heal”, the damage may be enough to cause rust. Zinc-rich “galvanizing repair” paints are supposed to be used to coat the damaged areas, but I would suggest that the zinc in such paints simply does not bond to the underlying steel in the same way that molten zinc does.
Caveat No. 2: There are a variety of zinc coatings and hot-dipped should be the only one accepted under these circumstances. That being said, the quality control over the amount of zinc deposited is poor which results in uneven applications (possibly even coatings that do not meet standard specifications). A high-performance coating rep has told me that he would prefer to specify a zinc-rich primer on properly prepared bare metal rather than a primer for galvanized steel because he really doesn’t know what he will get from the shop.
Caveat No. 3: Different components in an assembly may have different standards for galvanizing. If someone get fasteners with an electroplated zinc coating, these will not have the same life as adjacent hot-dipped galvanized items. The resulting corrosion will probably impact the performance of all components causing what may seem like premature failure.
A coating system over the galvanized substrate along with regular maintenance will almost certainly prolong the useful life of the assembly. How long depends on the quality of the galvanizing, surface preparation, and the coating system specified and the diligence of the maintenance.
Look at the installation required. It is best if you can maximize shop fabrication with entire assemblies galvanized after fabrication. Shop application of coatings is also preferred. All of this will have to be touched up (or “repaired”) in the field after installation, but you can be better assured of proper preparation of substates (the key to a good coating system) if the coating is shop applied.
If you decide to rely on the paint finish to protect the steel, use a high performance system with a good zinc-rich primer (this is not the stuff you get at Home Depot), and it may be more expensive than specifying galvanizing. A high-performance coating system may, in fact, be a better choice than galvanizing in some circumstances. Specify proper preparation for the substrate and then cross your fingers. Any damage to the coating that gets through to the steel (intentional or otherwise) will probably result in a coating “failure” due to corrosion of the the underlying substrate.
At the end of the day, the visual quality of a painted finish may be preferred. If the paint system is conceived of and specified as more of a “decorative” finish, you may be specifying a maintenance issue for your client. A good high performance coating system can work with the galvanized substrates to improve the protection of the underlying substrate, reducing required maintenance over the life of the building.