Wood finishes for outdoor wood need to be flexable. In racing sail boats when wood is encapsulated with epoxy as with the W.E.S.T system(wood epoxy saturation technique) wood movement is always taken into account. Use an epoxy that is too hard and the wood "checks" or cracks even when it's covered with fiberglass.
My only concern with the por 15 paint would be it's ability to expand and contract with the wood movement.
This is a two edge sword. Hard finishes resist scratches but don't move, flexable finishes move but scratch more easily.
My choice for bed paint would be the original tar based for a black bed and a marine grade varnish for a clear coat.
Both require periodic refinishing because of scratches and that's why the metal strips were designed to be higher than the wood and therefore take the majority of the scratches.
Put as hard a finish as possible on the metal.

IMO
John Atherton
Wichita Forest Woodworks

I know you believe you understand what you think I said, but I'm not sure that you realize that what you heard is not what I meant!