Actually it will hold up fine as far as durability and corrosion protection. It is just the finish that goes "chalky" with extended exposure to UV.
I have been using POR-15 for well over 10 years now and I am surprised at how well it holds up in even some of the most harsh environment. If you look I have not put the POR-15 anywhere on the top side of the trailer, only the underside (with the exception of where the wood planks will be). The top side is getting a couple coats of etching primer followed by single stage urethane automotive paint so it will withstand the UV.
Mike.