How come bumpers don't hold their shine?

[CBWhite]

I replaced my original rear bumper about 5-6 years ago because the paint had become dull and faded badly. I found one at the salvage yard that was in perfect condition paint wise. I mean once i got it washed and waxed up it was looking beautiful. Now it's beginning to get that dull faded look. I did everything that i thought i needed to do to keep it looking good but no luck. What is going on? Why the bumpers?

[idrivejunk]

The same amount of sun rays hit the plastic and metal parts of the car, but plastic doesn't transmit heat as well as metal so the paint cooks. At least thats my theory. Also, bumper plastic is porous so things can come up from underneath the paint. Consider also that GM applies as little paint as possible, and that the mixture used for bumpers may have had different additives. Same with spoilers, cladding, moldings, etc. In any case, 97-03 paint has outlived its original expectations so if any of it still looks good thats bonus time at this point. Even if garaged, the best bumper tops are starting to look bad.

[Scottydoggs]

sand it with some 2500 grit paper then shoot it with clear coat. if theres still color it will look a lot better, best to take it off to clear it.

[Turbocharged400sbc]

the additives that make the paints flexible for urethane body components degrade with UV exposure, they also lose their flex.