If panels have a little bit of lichen or moss, it is usually scrubbed off during cleaning. However if there is a lot of lichen and moss, it may need to be chemically treated. This involves spraying the area with a biodegradable bio-cide that kills the algae, lichen and moss. It begins working immediately but could take up to 3-12 months for it to die, breakdown and be washed away by the wind and rain. *No scrubbing and no water blasting