I have used this product for about 9 years. It definitely works. If you're not happy with results its user error. I know, because it took me a while to figure it out. For the most part, after using Corte-Clean for the first time, I realized I needed another bag to get really good results. Since I started out with one bag, and was too cheap to order another, I tried to stretch the " dosage " and thats where user error comes in. I have Trex deck and after a Cleveland winter, the black spots covered pretty much the whole deck needing a concentrated cleaning. In which case, you go through one bag really quick.