With all the holes drilled, I made up a solution of epoxy and microbaloons to fill the holes. Once cured and sanded, I'll drill them out to take the bolts for fins and straps. Just tried to use a roller to apply a coat of epoxy to the test board - just to see how easy it is before I coat the real board. As it turns out it'll probably be the easiest part of the process. I rested the test board on screws so that I can coat both sides in one go. On the real board I'll be leaving it topside-down, to avoid any roughness or stippling from the screws supporting the underside.