Brown and gray caps from around 2011-2014 all seem to have this problem no matter what mfg. Mine did this this on the front and rear. The mfg paid to paint the front after it was a little over 2 years old. ($1500.) I just paid to do the rear myself ($1050.) The mfg furnished the new decals. I tried the Zep thing. It started to crack and flake off after a year. It was a mess to clean off. Tried buffing, that only lasted a couple months. The only real cure is to paint it. Mine were done with a base/clear coat automotive finish. It's holding up very well. Hardest part is finding a shop with a big enough booth to paint it. I used a shop that paints trucks and buses.
B.O.