The reason you fill the weave with epoxy is because it the strongest coating and you already have a base of epoxy on there. The liquids epoxy / varnish are approximately the same weight. The reason varnish is used on the final exterior is because it gives UV protection, it's easy to apply and control and easily renuable.

If you need to fill a weave and you now have varnish on there you might as well continure with varnish or strip the varnish off and fill with epoxy and build up a varnish coating after that. In other words, there's no advantage to adding epoxy over varnish.