We receive a lot of enquiries regarding stubborn black spot that customers have tried to remove themselves in the past but simply cannot get them to shift. Unfortunately, black spot is a form of lichen, and it grows into the stone below the surface, some surfaces suffer with it more than others such as sandstone which is very porous along with concrete and various other surfaces. Chemicals are a MUST to remove black spot, occasionally steam cleaning is an option especially if the use of chemicals is restricted, i.e. if the job is next to a water way. However, even with steam a lot of black spots can stay in the surface which is why chemicals are the best answer where they can be used, unfortunately these chemicals are controlled substances and restricted to sale only to professional service providers.