Try getting a plastic safe all purpose cleaner. Gtechniq W5 is pretty intense and cleans very well. Spray it on the fender and take a stiff bristle brush and give it a good scrubby scrub. If it doesn't come all the way out, just repeat a couple of times, working in small sections. Wash, rinse and dry.Just wondering if anyone else is having this issue or not. I have the plastic fender flares with the sasquatch package so I expected mud and rocks to sling up but i can't seem to get them cleaned. I haven't taken it "off-roading" really. All the mud I've picked up from my short dirt driveway and anything on the highway. Washed it first with a soft wash mitt, soap and water and it dried like the photos. I tried a harder sponge and scrubbed it pretty hard and today I took it to a car wash and used the pressure washer and still no change. The dark spot on the fender is just a wet spot I tried to clean with my thumb. It dried like everything else. Thanks in advance.
For longer protection, try applying Gtechniq C4 Trim Restorer, great product, rejuvenates plastics, darkens finish, provides a level of protection and water repellency. Some suggested Trim Solution. That is also a good product, very messy though. Have to be more diligent in it's application. It uses suspended charcoal powder that soaks in to the plastic pores to bring it back to life. However I would recommend trying to remove the stains first. Trim Solution is a dressing, it doesn't remove stains.