avation, don't waste time with a vinyl insert, use undercoating, line-x, rino, or something instead. You'll get a better rust protection bond. BRD HNTER used Hurculiner, ask him what he thinks. TT uses some other type of undercoating material covered with paint (you could use a color match paint) on their high end custom stuff. Spray-ons don't squeak, make noise, cause rub spots, etc., and make the bond we all need to prevent rust from the top side.