Not required. We used masking tape and varnish on lead foil which had no sticky side. What a performance that was ?, clean the window with meths on a rag, mask the area, apply varnish wait for it to go tacky, then apply foil, then varnish again. When this stuff came out, it had adhesive on one side, so it was just a case of clean, stick and done, no varnish required. One customer complained, no varnish, went back, varnished it (Customer is always right, even when they are wrong) week later had to remove all foil and varnish (Good old window scraper) someone put a key across the foil, nice gouge out of it. He was paying by the hour? Never did use varnish again, ever.