fz,

It all depends on what quality you are after - good looks, light weight, long lasting. You could just brush a coat or 2 of colored dope on and go fly. Would be very light, wouldn't look so great or last too long. If you want lasting good looks you have to do something to harden and stabilize the soft balsa surface. This is why fiberglass base is used. The heaviest solution but no limit to how good looking and long lasting you can make it. The old silk and dope, or now days modern polyester fabric like Sig Koverall and polyurethane or epoxy paints. Not a rock hard surface but does the main job of stabilizing the wood grain. Wood grain expands and contracts with changes in the weather. This type of finish is a great compromise. Light weight and long lasting.

Multiflyer