I have a 5-year-old Elk composite deck. I was beside myself when black mold stained my new composite deck, for the first 2 years, which is 10’ off the ground and 27’ X 12’. I tried Clorox the first year, which did nothing. I then found out about Corte Clean. I have since been using this product spring and fall. Last year, I forgot to do the fall cleaning. To my surprise, the deck is still clean except for a few small stains. The deck now stays clean for longer & longer. Last year, when I removed the patio furniture, orange rusty water spilled all over the deck from one of the chairs. I thought this would never come out. I mix up a little Corte Clean, sprayed it on the affected area, and the stains came right out with a little brushing. I have a tip for those that use a cheap pump sprayer. Use a strainer to filter out any undissolved Corte Clean to prevent clogging. I selfishly wish you continued success, as I will be needing Corte Clean in the future and just bought more today. If I could give 10 stars, I would.