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I am a Painting Contractor, i do lot's of exterior doors... the best product i have found is Mccloskey's Man-0-War spar varnish, Use a sanding sealer first!
With rare exception, most of todays varnishes are poly based - even the exterior spar. Epoxy is more hardy but if you ever have to refinish, getting the old off is all but impossible.
The finishing process is 100% dependent on what type of wood you use. Oak, Pine, Ash?
Oak and Ash are very porous and will turn black if you don't fill those pours. Pine is oily and will not absord a finish as much as a hard wood.
There are tons of threads here on FTE about bed finishing. Unfortunately you don't have enough posts to use the search.
I'll post a couple for you. But also, MAR-K, who makes beds here in the USA, has done a number of studies on products and different woods. I would recommend you log onto their site and read up on those as well.
Just FYI, the OEM Hard Pine planks installed in the beds by Ford, were painted the same color as the truck with the same automotive finish the truck recieved.
PS Just as an experiment, I finished the Sedan Trunk on my trailer with a Clear water based Acrylic. This was my first experience with it. It was a little different to apply (and I can't describe why it just was) but it is absolutely CRYSTAL clear, is rated for exterior use, dried VERY hard, and I put it on Oak and have no blackening - so far - been on there about 6 months.
I'm in the middle of refinishing my bed wood. I saw a new product by either Horkey or K-mark in Classic Trucks mag that they recently developed and tested. Of course they say it's the best, but I was wondering if anyone has tried it. I emailed Eastwood Restortation about using a 2k urethane clear and they replied that Z Spar was a better choice. I finished it in polyurethane and it turned yellow on me and does not look good on red oak. I've spent over 10 hours of labor sanding it off!!
Be sure that anything "Poly" that you use is designated for "exterior use." Moisture is not as much of a problem for Poly anything as much as UV from the sun. Poly products not supplimented with a UV protectant will decompose fairly rapidly.
I used Mar-K's recommendation of Pelucid and Helmsman Spar Urethane. I put it on over two years ago and it still looks great! Eventually, I'll get the truck painted and assembled. Then I'll take the boards off of the shelf and put them on the truck.
I have already stained and sealed my red oak boards with a red mahogany finish. I would like them to end up really glossy. Would finishing them with an automotive clear really work?