My Trex deck is about 12 years old. I did not know that it could get mold... black spots. Trex was being marketed as “no maintence decking” at the time, if I recall correctly. So what was happening and how would I address the problem? A google search eventually led me to Corte Clean and after trying it and seeing the results, I am just so grateful to have discovered this product! I have been using it for years. I live in the northeast and have discovered that the best time for me to clean the deck is at the end of May, when the morning reaches at least 65 degrees and is sunny... but the humidity is low. I can get it done much more quickly. I have also started using a more diluted cleaning in the Fall. The results are that the deck is not as problematic in the Spring. This is a wonderful product and the service is always very friendly and helpful when I call. ALWAYS buy a fresh bag before cleaning. If you don’t, it will not work as well. And if you do, it will do the job VERY well!