Rosemary asked: I manage a historical building, and the light gray granite steps are attractive to the local skateboarders. They have waxed the steps, leaving a black residue that I have been unable to remove. I have tried various chemical cleaners and a poultice of peroxide and baby powder, but cannot lift the black stain/residue. Do you have any suggestions?
Skateboard wax is a mixture of wax and oil that is applied to the surface to make it slick and minimize friction as the skateboard slides across. When this wax gets hot, it can melt and move into the pores of the granite. This is what causes the dark stains. Since it is below the surface, they are very difficult to remove. Removing the wax from the surface requires a pressure cleaner that can heat water to 200° or higher. In most cases, it is best to hire a professional who is experienced with granite to complete the job. Keep in mind that if the stains have gone far below the surface, they may be impossible to remove. However, it should be lighter in color at the least.
To prevent skateboarders from causing additional damage, consider installing skate stoppers. These are specially installed surfaces that are coarse and difficult for skaters to cross.