We use very thin light poly varnish finishes on balsa discus launch gliders (DLG). The trick is to wipe it on with a dunked and squeezed out paper towel, not a brush, and wipe it off again with a dry one. A few layers of that is super light, and yet prevents warping of the balsa skins with humidity changes. Sorry to bring in just a non-bow comment, all I can do at this point, but thought it might help. Also as a former boatbuilder, the best polyurthane varnish for really svere service is/was Z-spar Captains 1015.