These FRP panels are waterproof and somewhat UV resistant. I'll finish paint them to further protect them from UV. These are white polyester based fiberglass reinforced panels from Menards. Other hardware stores also stock this, but you want to be careful to make sure what the panels are made of. Basically these are the same material as fiberglass cloth with polyester resin, which is what I am using to reinforce the inside. Before I finish the outside, I will also grind, fiberglass, and sand down the few seams I have on the outside. My goal is to keep the outside as free of bodywork as possible, and so far it is working out pretty well. If I can get the outside as smooth as I want, I plan on painting it Crystal Granite to match my Gladiator. If I can't get it smooth enough, there's always the textured black most hardtop Jeeps have.
One note before anyone tries this though. The FRP sheets are pretty stiff and can only be bent so far before buckling. It will take some trial and error to find what radius works for your application. Good luck!