It's hard to determine for me exactly. There is really no flaking just a general fade happening. Rewind this about 8 years ago, the 3rd brake light burned out and I took it to Ford since it was under warranty. Some idiot must have used a flat blade screw driver, and left a 1/2 inch scratch on the roof that they didn't tell me about. Before I left, I checked the repair and noticed the scratch. They had me take it to their body shop and appeared to do a good job. I noticed it starting to fade in the past year.
I am guessing there is some paint fade, but mostly clear coat wearing off.