Been building Byron, Iron Bay models for over twenty years now. As above use polyester resin when attaching anything to the fiberglass. Use epoxy or water based polyurathane to attach the glass cloth to the flying surfaces if sheeted. I would only use epoxy resin (ZAP finishing resin) if glassing directly over the foam. Do not thin Zap resin with alcohol! it will turn it to rubber. Ask me how I know this? Made a couple of gear doors using said method. thought I had a couple of Rubber Maid plastic doors. I then made new ones without the thinner and they came out perfectly stiff and hard. Sanded a whole lot better...LOL
And when attaching anything to inside glass , make sure you rough it up with 40-80 grit sand paper first, then clean with white vinegar. The acid in the vinegar will clean all the parting agent off the glass. Reecived this advice years ago from the builders at the factory. Good luck on your project. They fly great!