In the new video from Van's showing how to fiberglass the front of the canopy (fairing)(great by the way), they mention using an filler primer and rolling it on for the finishing step to fill any remaining small pinholes. They don't mention a brand of filler. Looking around here, is K36 the recommendation?
All primer-fillers (aka high-build primer, primer-surfacer, and often primer-sealer) are a base liquid suspending a solid bulk media. Most of the good primer-surfacers these days are acrylic urethane base. The common bulk media is talc or similar.
Years ago, the base was polyester. In the immortal words of Monty Python, "Run away, run away!"
Any solid (or liquid that turns solid) that you can work
down into the pinhole will satisfy the basic requirement. We have good builders here who swear by drywall compound, and I don't doubt river mud would work well too. The question is how will the chosen filler (1) react long term with the substrate and the overcoats, and (2) hold up to environmental conditions, mostly a matter of heat and moisture.
Note that in the above, the material is not used as a surface coating. It is not a
layer in the stack of primers and paints. Its all sanded off, leaving just the material down in the holes.
I quit using acrylic urethane surfacers for pinhole work some time ago. It's good stuff, but they all cure in the usual manner; the solvent evaporates out, then cross-linking hardens the material. Although it's not difficult to work the primer down into the pinholes (squeegee, roller, old credit card, whatever), I find that as the solvent evaporates, the pinholes tend to re-appear as little depressions in the surface, or re-open. So you let it cure, reapply, sand again, reapply, sand again, etc. I prefer an epoxy wipe, as there is no shrinkage during cure, no long term chemical or environmental issues, and it's a one-shot application.
I do use a lot of high-build primer for its intended purpose, as a final build coat prior to topcoat. Spray a contrasting epoxy primer, then spray a few mils of high build, then block sand to eliminate the last little fine ripples in the surface.