So I tried to clay a small portion of the fender. it took of a bit, but still a lot remained.
I contacted XPEL and they recommended using Westley's Bleche-Wite (yes, the tire cleaner) to remove the hazing. I went out and bought a bottle and tried it on a very small spot and it worked great.
I'm a little hesitant to use it since it's some pretty strong stuff. but I may give it a try going slow and careful.
I'll post the results if/when I get the guts to try it.
2008 E90 M3 (IB, Premium, Tech, Cold Weather packages, PDC, 6MT, 19")