[63 & 64 Bowties]

I'm no expert, so this free advise is worth exactly what you paid for it.
But, I've been told by two different "experts" to not seal wood completely (top, bottom, sides) with varnish, urethane, resin, etc.
I had issues with a wood sign that I had custom hand painted where the marine spar varnish (which I applied) delaminated. I was told that if you completely seal any wood, you aren't leaving an escape for moisture that's naturally in wood, and you're not leaving room for temperature expansion and contraction. With bed wood, if you're using varnish, you may be better off not sealing the bottom.
*disclaimer* this is just second-hand info, but told to me by some people who I trust.