I would think that some mild dish soap, or basic car wash should be sufficient. Harley also makes a denim paint cleaner. Probably don't want anything that will add too much gloss to the finish. I personally use Meguiar's Gold Class wash. There is no wax in it, and it has a paint conditioner. I've used it on my matte black pipes too, and it doesn't change the appearance of the finish. I just minimize the amount of rubbing to avoid "polishing" the surface.

 

Well, depending on how bad the marking is you might want to resort to simply a repaint. It is a matte finish so it will be a snap.

I heard that what they did (or didn't do in this case) is simply not put a clear finish over the black colour coat.

If it is just shiny, but still black, you can buy a can of matter clear and spray the whole fender. That will bring back the matte finish as well (since it's a clear coat) protect it. (you won't get the worn in look that harley advertises).

I build custom furniture and people like the look of milk paint (shaker style) but it is so susceptible to wear. Some like it but for those that want it to stay the same? I use matter clear lacquer over it.

I also do custom metal work and whenever someone wants a raw steel look, I use krylon matter clear. Works like a charm and prevents the raw steel from rusting.

I'm seriously thinking of taking all the tins off my nightster this winter and matting it all out like the iron. Will confuse the heck out of people if they see what they think is an iron then see the 1200 on the side. It should be easy since all I have to do is put a matter clear over the gloss (after a little rub down with 2000 grit that is, but that might even be enough to take the gloss off).

It's funny really. Back in my teens I worked in a car restoration shop and the theme of the day was make it as shiny as humanly possible. My boss is probably rolling in his grave hearing me talk of matting my bike out lol.....

 

The ONLY way to fix the problem, is to re-paint the fender. The more you rub (wash) denim paint, the shiner it gets. Harley does sell the Denim clear coat in a rattle can.