High build and other 2 part POLY primers may be porous, full of talc or other moisture absorbing fillers but Epoxy Primer is WATER PROOF as in water will not penetrate the bond of the surface molecules. It's why BOAT BUILDERS use it to seal their fiberglass...
So I'd choose a good epoxy primer over clear any day. Especially if I had plans to come back and do more body work on the truck. The clear would have to all be removed where as the EP would merely need a good scratch with 80- 180 grit and carry on as normal with filler or what ever.
My understanding is SPI black epoxy has some UV inhibitors in it and would last for two or three years before chalking up and even then just would need sanded down some before doing work over it.
