At least one of the books I'm using now states: "Don't worry about the white spots of cloth showing . . . they'll wet out with the varnish." On my S&Gs, heavy as they are, there are spots I can see in the sunlight where the weave pattern is visible. Is that what I can expect at every location where I can see the weave now on the current project? Should I plan to add another coat of epoxy, then sand again?

Yes, about twenty years ago I had the same problem.

The initial fill coats follow the rough texture of the glass weave, so you'll never know if you have enough fill coats if just keep sanding to until you want to varnish unless you apply a lot. How much is enough? How thick are your fill coats?

To consistently avoid this, I do all my glassing work, plus 3 fill coats, join hull/deck, glass external seams and stems before sanding.

Then I rough sand and feather seam/stems with 60 grit. Any place I start to see little white dots, I know I'm starting to hit glass weave tops, and I move on the sand elsewhere. This creates a level uniform surface.

I apply TWO new fill coats. ONE is not enough to ensure you won't be right back at the glass level.

The new coats go on very smoothly, with System Three epoxy, and I sand with 120 then 220 and varnish.

I no longer have to wonder if I'll hit the glass.

Rob