If you had 3 hard coats of D2T on it, I don't think any other combination of coatings would have fared any better. Balsa has lots of air inside - it expanded.
I use Aluminite RC3 Urethane resin with Micro balloons.
I measure the resin 1 to 1 with a scale, I add the micro balloons
by volume to about 80% of each container before mixing containers together.