I have a Trex deck about 6 years old. After about one year I notices alot of black spots on the deck. Did some research and determined it was black mold. Googled the problem to get a solution and called Trex as well . I didn't want to use all the bleach products out their which I felt were unsafe after reading their warnings on the packages in the home stores. Then i went through the list of products that Trex recommended. One of them was Corte-Clean. I ordered 3 containers as they were not sold in bags at that time. I really had to work at it as the mold was really bad. After 2 applications it was much better but not as good as new. I called Corte-*Clean and spoke to Greg, who I believe is the owner. He made some suggestions and the next weekend I gave it one more shot. Big improvement and I was very happy. The next year the mold was back but not as bad as year one. I did 2 coats this time and I was very satisfied. Don't believe anyone who tells you Trex is "NO " maintenance. Fast forward to 2 years ago. I wasn't feeling 100% and did just one application in May. Deck came out really good and I was set for the season. Last year we had a million things going on and only did one application to half the deck. I just went outside today to look at the condition of the deck. I cannot believe their is no mold whatsoever on either half of the deck. The deck is a little dirty but not really that bad considering. I just called Corte Clean and ordered more product for this year. Greg answer ted the phone and we spoke for a while. He said it must be the product has finally gotten into the TREX and gets reactivated when moisture hits it. He gave me a few tips from things they have learned in the last few years, and I look forward to an easy cleaning this year. My advice is BUY this product and follow the instructions to the letter. Call the company if you have any questions or problems and only use the product if the expiration date has not been exceeded. Good Luck Don Heide -New York