Hello,
What is best method for cleaning and keeping black billboards looking new
Thanks

Microfiber towel using water with a drop of dish soap in it.

Ok to use a little mineral spirits on a microfiber towel to remove anything tougher like adhesive.

use a glass coating over them, I use it on my 3m stripe on my 70 and on the hood and also used it on my stain hood on hellcat.. makes water spots, birdshot and debris come off like nothing..

kamikaze nano coating, there are others, I prefer this, can be found at esoteric car care on Ohio...

I used Meguiars paint cleaner on a faded black stripe. Lightly rub back and forth, not in a circular motion. It brought the color right back. For cleaning on my 340 Duster, I use :Cinch" to remove greasy finger prints and simple quick detailer to clean them. The quick detailer leaves the matte black stripes non glossy and offers some protection as well.