I don't think you'll be happy with the result if you don't varnish. I decided not to varnish the inside of one of my strip canoes thinking the varnish was mostly cosmetic. After several seasons several white spots appeared - from coin sized to 2 or 3 inches across. I think this is where the glass is separating from the wood. Also, I think the varnish coating also offers physical protection for the fiberglass and resin.