I just sprayed the length of my dually from the part line on the door down to the bottom with a SEM protective coating. It comes in 2 textured flavors and is clear. You can paint over it or get one that you just spray on and leave. These were spray cans at about $18. 00 per can. Easy to use and you can be done in an hour. 3M also has similar products. Contact your auto paint store. The product works great , I color matched mine and clear coated over it like the factory. Good luck.
Mark