For what it's worth I used Minwax Helmans Spar Urethane that I got at home depot. The bed was oak and I used the truck as a daily driver. After about 6 months (including an east Tennessee winter) signs of black mold, a bit up cupping and cracking on the ends of the planks appeared. I think I had applied 3 coats.
Nothing was hauled in the bed during this time.
I think next time around I will use yellow pine like the original AD trucks and leave it bare (the next truck will only be a weekend driver).
