By Hendri (Force From DR.com Forums)
I use automotive Tar Remover to remove black scuff marks (from boots etc.) and other dirt marks on my plastics, specifically the white number plates. I couldn't find anything else that removed the stubborn marks, but as I contemplated cleaners, I noticed the Tar Remover. Some brands are labeled "Bug and Tar Remover." It evaporates quickly, so I sprayed a bit on a small part of a cloth, then pressed on the marks, and rubbed quickly in short strokes.