Go to a good new stand and get a copy of Wooden Boat magazine. Read up on different spar varnishes. Boats deal with all the same elements as a truck bed does but to greater degree. Therefore, I rely on marine products when dealing with wood, water, sun and long term preservation. Depending on your location go to a good boat yard for purchase.
Another source is
www.woodcraft.com. Great outfit of quality and knowledge.
