You do not need Balsarite to seal the wood. As far as I am concerned Balsarite is pure cr#@. You didn't say how you were going to finish the plane. If you are using iron on covering all you need is a smoothly sanded surface. You can use some cheap hair spray to help raise the grain and then give it a final sanding. Balsarite will muck up the surface and is almost impossible to sand. If you have evere tried to recover a plane that has this junk on it, you will curse it forever. About the only place Balsarite can be used IMO is around the engine bay.

Vince