: I am aware that any varnish over exposed fiberglass is a bad cosmetic flaw.

Varnish will also wetout glass touched by sandpaper and will hide the exposed glass however with age those varnished touched glass spots tend to develop shiny flecks in the epoxy. So I always apply a final coat of epoxy after fairing to cover the touched glass and fill the pocks (epoxy bubbles sanded in half). Then I take the kayak paddling for a month while waiting for a complete cure. Then a light wet sanding with a shot of dishwashing detergent in the sanding water and a good rinse with a hose is all of the prep needed prior to varnishing.