Several scale guys I know use a final coat of polyurethane. As with any finish test the system you plan to use on a piece of solartex covered scrap balsa first. I discovered that I can achieve a much sharper masked line by first shooting some clear BEFORE the color coat to seal the tape edges and prevent undermining into the fabric weave. My original plan was to final spray my plane with Minwax Helmsman Spar Urethane but the white on my test sample turned beige! I guess there is some tint in the stuff for enhancing the look of wood? Disaster averted.... I like the Duplicolor because it dries fast, remains flexible and comes in some really beautiful colors. Check the touch-up section of your neighborhood auto parts store.