We have been using corte clean for many years on our fiberon deck which had major issues with black spotting from the get-go. It looked like the entire deck of 900 square feet had been covered with black spots! It was very disheartening because we spent so much money. Fiberon was no help. They sent us deck wash with a disclaimer saying they would not be responsible if the product damaged the deck. Why would anyone use it! I researched and found corte clean which I have been using for years. It is the only product that will clean the fiberon to a reasonable appearance. Definitely follow the directions and let the corte clean work for you! We clean the deck in the fall and leave the white film on so it continues to clean all winter. We do it again in the spring! It is, by far, the best product on the market! Feel free to call them. The owner was very helpful with questions I had. Again, if you have a Fiberon decking, you need this product!!!