When removing graffiti, the surface to be treated is particularly important. The more porous the surface, the more difficult it is to remove graffiti. Porous and unprotected substrates, such as many types of natural stone, as well as sandstone, travertine or limestone, are a challenge for any graffiti removal process.  The graffiti paint soaks deeply into the substrate through the many and usually deep pores. The surface can usually be cleaned well but the shadows remain. Such sensitive surfaces must be treated with graffiti protection if there is a latent danger of graffiti.