apply the glass , then sand that perfectly Flat , fill , then sand more , then paint it ( more sanding) ...
My last build was glassed, primed, and then covered with a urethane bed liner. The sanding was minimal. Now, this was over wood -–not foam-- that had been checked for high and low spots, but plywood is smooth.
I sanded the gloss off the fiberglass surface with a random-orbital sander just to give some tooth for the paint. That was no more than a half-hour. There was no need for filling after this sanding. The epoxy filled the weave. And no "ichies" from the glass since I was not chewing into it.
That was followed by priming, which showed a few low spots.
I filled those places with automotive spot filler. Ten minutes with the orbital? Here's the final surface:
You may think that the texture on the final finish covered blemishes such as cloth weave but that's not true. I was not relying on it to solve issues and I don't think it would have.
Not a lot of sanding.
Tony
P.S. One thing I have been scratching my head about is the PMF texture issue. PMF came out of the canvass covered wood canoe era (I'm guessing '20s through the '60s). Fine boats but they do need some TLC.
You'll note that the canvass surface on those finally made boats show no weave. They have a process of dealing with it. (Which includes sanding and filling.)