I don't know about the rubberized undercoating. The advice I gave you on brush on truckbox liner is knowledge from a magazine. My first hotrod, the 31 Desoto coupe had steel fenders all around. The front fenders were a pair of model A fenders that were so rough a restorer gave them to me, and the back fenders were steel trailer fenders that I molded and bobbed. By the time I had rebeaded and patched the front fenders, I had close to 150 hours in the 2 front fenders alone. My next rod, the 27 roadster had no front fenders and glass fenders on the rear. I never put anything on those glass fenders, and I never had a problem, but "popular knowledge" has it that you should put a resilient inner lining (coating) on glass fenders to absorb the shock of stones flying up off the tire and hitting them.