A water-viscosity epoxy as a seal coat...
I was asked to fabricate and install larger tire clearance blisters in a set of Falco gear doors. First task was to fabricate a form defining the shape of the desired blisters. Easy enough; ordinary 2" pink foam from the big box store, a little micro, and one wet coat of System Three Clear Coat.
When cured, sand it slick without breaking through the skin.
It was the waxed twice and sprayed with PVA. Two plies of 9oz 8-harness were laid up over this form, cured, and popped off. Another squirt of PVA, another layup, and I have two blisters with excellent shape.
Next task was cutting the old small blisters out of the gear doors. The new blisters were bonded into place, and two more plies of 9oz were laid over the assembly, followed by sanding to fair it all together. One coat of micro for fine contour, finish sand, then bring out the epoxy coat to seal it all. End result is a 4 ply blister on a honeycomb door panel, nice and light because there isn't a whole lot of filler on it:
The sealed doors were sanded slick, just like the pink foam form. I don't remember if I shot them with PPG K-36 or not. I would have if they had shown some very fine ripples or waves when looking across the surface at a low angle. That's the purpose of filler-primer...flattening any surface contour flaw by block sanding. However, if all the work up to that point was done with an eye toward not creating waves in the first place, it's possible to go directly to epoxy primer and finish coats.