I had a small hair line crack on either end of the cabinets in the end cap.
Before you change the color of the cap you will have to repair the crack in such a way that it does not just reappear. I removed the cabinet and fiber glassed the back side o the crack. While the patches of fiber glass were small, about 3 x 5 in. I used a couple of layers to be sure of structural strength across the crack. If you can get to the back of the crack consider this approach. If you can not and are going to paint the end cap you could do the repair on the surface if you know how to work with fiberglass and body filler.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles