What I have found is that if you sand into glass with a coarse grit, often the weave does not want to wet out or wet our well. If you wet sand it with very fine grit like 300 going up to 600 you will see the glass disappear. The advantage of wet sanding is that you are wetting out as you sand and can see how it wets out. Glass doesn't care if it's varnish, epoxy or water. Sometimes if you're sanding in preparation for varnish and you expose the weave, wet sand the area very fine and you can see it wet out. Better to do that than have a bad surprise if the varnish doesn't wet it out.

I also learned that if I'm in doubt about wether glass needs another coat or not, I use epoxy. It's harder, weighs the same and is ready to sand in days.