The first bed boards I treated in 1997 were pine boards stained with MinWax ebony/black stain, and then 6 coats of 50% diluted MinWax semi-gloss Helmsman Spar Urethane varnish on all six sides (completely dried and then lightly sanded between coats). It has lasted fine and only started showing a little "checking" in the last two years. The truck/bed has been mostly living in a non-heated garage
Any sealer-finish is going to fail if/when any part of the "seal" on the wood develops any cracks that allow moisture to transfer in/out of the wood.
