I haven't done it much with balsa, but from my experience with metal, if there is slight pitting, you can use an etching epoxy primer. The epoxy works to fill in the pits (just small pits mind you) and can be sanded then gone over in enamel, and you wouldn't be able to tell a difference. For balsa, the question is, are you intending to fly this plane? I am assuming that this is for static display. I know a gentleman that did a Dauntless who used bondo (sparingly), then sanded smooth, primed it and used an enamel paint finished with a clear coat. I wish I had a picture because it looked stunning! You can get this at Home Depot though, the product is from Dap and is called Fast N' Final lightweight Spackling. This works even better than bondo putty. And it's much lighter weight.