I doubt you will be able to get rid of the grain of the balsa. On top of that it will take a bunch of filler, sanding and so on. You will also run the risk of cracking when the balsa absorbs moisture and then dries out. Koveral and poly seem like the best option to me with the least amount of work. It will take a lot less filler to fill the covering weave than it would take to fill the basa grain in my opinion.