Filling the grain is a lot of work period, there's no getting around it. You either need to use fiberglass cloth or silkspan to cover the balsa first and then use epoxy, polyester resin or a water based polyurethane to fill the weave. I've also heard of Klass Kote clear being used on bare wood with good results. Any method that you use though, a lot of sanding is needed.

FB