Multiple layers of either wood glue or white glue (PVA glue) mixed with a bit of water to thin it, and brushed on works. Gesso or Mod Podge work similarly.
For something a bit stronger, you can go to polyester or epoxy resin. I believe polyester resin will dissolve floral foam (floral foam, aka "Oasis foam" is made from phenolic resin, curiously, the same stuff used to make billiard balls. The More You Know!™) So you first need to paint surface in a solid layer of acrylic paint. Probably at least 2 coats. Both of these resins are commonly used in the aircraft industry. They are lightweight and strong. Even so, it won't be super durable without reinforcement, so, just try not to have any major crashes.
Out of curiosity, exactly what prop are you trying to make?