Outdoor stone surfaces face a combination of challenges that indoor surfaces never encounter: UV exposure, frost cycles, rain, biological growth, atmospheric pollution, and heavy soiling from leaves, bird droppings, and environmental debris. Regular outdoor cleaning and maintenance is essential to preserve both the appearance and structural integrity of outdoor stone. Outdoor cleaning combines several treatments: removal of biological growth (algae, moss, lichen), removal of atmospheric soiling, and optionally re-sealing the surface.