For the metal powder coated wiper arms, when the paint looks faded I used to use something like KAIO to try to clean up the best I can, but you never get that black back again. I have had very mixed results trying to abrasive polish powder coat in the past. Sometime within the past year I tried using Solutions Finish on the powder coated paint. I was working on the windshield cowling and got some on the wiper arms by accident. I wiped off that spot and it looked really good. I waited to see if it would go back to a faded dry look after ample dry time, but it didn`t, so I did the whole wiper arm.
So Solutions Finish does something to revive powder coated paint (black of course). I suspect that it may fade quicker than the treated plastic will, but I`ve used it a few times now to make old tired looking wiper arms look much better. I guess I`ll see how they held up when they come back in 6-12 months.