Fins: I use thin CA to seal/harden. Sometimes I paper with CA saturation. If I'm just filling sealed fins, I will either use coats of filler primer (Rusto Auto filler), or any of the following: Carpenter's Wood Filler (CWF), Bondo Spot/Glazing Putty, Tamiya putty, etc.
I don't seal my paper body tubes per se, though the filler primer I spray on as my first coat of "paint" seals the outside. Never bothered sealing the inside (but I would swab thin CA if I wanted to do that).
Don't use much plywood.
I'm all LPR or very light MPR, so bigger birds would probably require different techniques, or additional techniques (epoxy etc.).
Me I use thin CA to harden and seal balsa nosecones, and exposed edges of LPR body tubes... If it's glued to a coupler, then that will be seal enough for me. I paper my fins (balsa and thinner ply fins) then use thin CA squeegeed to seal all the edges except the root.
For thicker ply fins, and MPR/HPR body tubes, I'll be sealing them with the water based Minwax Sanding Sealer (if I can find it here).