Read up on how fine wood kayaks and canoes are finished. Buy 2-part epoxy resin by the half or full gallon, sand the wood smooth then seal with the epoxy, then sand before your choice of varnish, etc. Wish I'd known about kayak techniques before I did my bed in varnish only (never again!). With a decent wood the epoxy will encapsulate and virtually eliminate any chance for moisture intrusion into the wood, ever again.
And when applying varnish, study those kayak techniques...foam brush,
always stroking into the wet edge. I built this kayak kit by clcboats.com, they're a good resource worth a look by anyone who wants a fine wood bed in their old truck. Then again, if a mirror shine isn't what you want...never mind.
