In my experience, using something like silkspan over the balsa will make filling much easier, faster and lighter than trying to fill the grain of the balsa itself.

I have also seen a person airbrush monokote with regular enamal paints and then apply a clear fuel proof sealer over the top. Looks very nice and is a very quick method of finsih that I may have to try sometime, although I am not sure how well it holds up over time, particulary given that monokote is not what you would call dimensionally stable.