I used to seal the wood, but I now just use Balsarite or Stix-it painted only on the areas where I iron the covering to the frame. I've never found a covering that will stick well to bare wood. If you prefer to seal the wood, such as around the engine area, Balsarite can be used over wood sealed with just about anything. I've used epoxy, Minwax (both urethane and acrylic), Mod Podge, Elmer's glue, Kilz, and thin CA. All worked well, but probably only added weight. Have fun.
Paul