Here's the riddle. The wood surface needs to be "stabilized". Yes you can fill the grain with dopes, primers and paints, but the wood can still expand and contract with temp and humidity changes. Balsa wood is very soft and can get dings and dents very easily even when the grain has been filled. Covering the balsa before painting will actually seal it and give it a protective barrier against the dings plus it will usually be a lighter weight finish to boot. It takes quite a bit of filler just to fill the grain on balsa not to mention the amount it takes to get a smooth finish. I prefer to use lite weight fiberglass cloth(.5 oz.)/ epoxy resin to seal solid balsa parts before finishing and painting.
2 Piece