The finish of the paint is only going to be as nice as the finish of the final coat of primer your painting over. If your happy with where your at now I'd wash it down real good with dish soap and water. Dry it off good.tape off everything you don't want paint on and cover all the areas that dirt could blow out of and get in your paint. Use some wax and grease remover on it tack it off and paint it. Single stage urethane isn't going to leave you alot of room for eror so make sure you got your gun set right dont spray it on to dry and don't put it on too heavy and run the paint. Enjoy! Lol