I also used Minwax Helmans Spar Semi-Gloss Urethane (7 coats - do not ask why, I do not know). I thinned it 50% on the first two coats in an attempt to get better "penetration". This seems to have worked pretty well (13 years). However, I have a heavy rubber bed mat and try to keep from damaging the surface seal of the varnish.
One of the most important things to be careful about when applying a varnish as a moisture sealer is to cover all six sides of the board (end edges are especially important and can suck-in a lot of varnish). Also, if there is ever any mechanical damage to any part of the board - damage that breaks through the seal, you will 100%-certain have expansion and contraction from changing internal moisture. Cracking is then assured.
