With all the parts milled and test-fit into the truck bed, the next step (and really the crux of the operation) was to finish each individual piece with an epoxy sealer and a marine-grade spar varnish. I applied the finish as follows: two coats of MultiWood Prime Epoxy Sealer, followed by four coats of Epifanes High Gloss Marine Varnish. The first coat of varnish I applied at 50% concentration, the second at 75%, the third at 90%, and the final coat I applied at full strength. High Test, in other words. I 36 hours betwwen coats of epoxy sealer, and 24 hours between coats of varnish. I sanded with 320-grit paper between each coat.
My hope is that this finish will last several years before it requires a fresh coat of varnish, although given the amount of abuse it will most likely suffer in the back of the truck (which lives outside year-round), your guess is as good as mine as to how long it will actually last.
Special thanks to Mr. Sean Clarke, for his article "A Durable Exterior Finish," which appeared in Fine Woodworking #179.