during regular washes on my JK plastics there was never an issue with just making it part of the regular wash. If you have mud and such that won't come out just use a soft bristle brush and that should take care of it. If the fenders start to look faded I had great results with Meguires black plastic restore https://www.walmart.com/ip/Meguiar-...MI9IKEncmX5QIVbx6tBh3GWgkgEAQYASABEgKe2fD_BwE
The top is not plastic like the fenders so it is much easier to clean no different than the rest of the body to be honest and it doesn't fade like the plastic. My 2011 JK with the Meguires put on occasionally made it look much newer, no one believed it was almost 10 years old when I traded it in for the JT.