Well, if you're using normal resin/fiberglass methods, your armor is already going to be pretty water resistant, aside from the paintjob. And that can be fixed with a few coats of a gloss (or matte) sealer of your choice. In fact, put on enough gloss coats onto any piece (even cardboard) and you'll probably be pretty good on getting rained on. Now, if we're talking full submersion, that might be a different story.