What you're talking about is the amount of weave showing. In my experience, if you find that the weave is showing everywhere, coat it with epoxy. As Bill said it's easier, faster and wiser. You will have a rock hard surface, so when you do sand the varnish for a re-coat, you will have a hard surface to sand to. Varnish sands away easy and you can sand right through it fast. One one boat I built, I did not re-coat with epoxy and I regretted it over the years. When I re-varnish, I have to be extremely cautious with sanding and more often than not, I expose fabric. If you have a few square inches on a curve somewhere with a tiny bit of weave showing, then just varnish.

The 2nd issue you refer to is; generally if the weave is showing under the epoxy, your epoxy was not warm enough to fully wet out the glass or... the glass was not a good epoxy compatible type. Normally under good circumstances, the weave does not show. I have an old boat with weave showing and I have just learned to use thin epoxy that's warm and I buy my fabric from people who sell kits and know the fabric is good for epoxy wet-out.

The bulk of glass sold everywhere is used for construction or reinforcement purposes and usually gets gel coat or paint over it.