We installed grey TwinFinish planks on our upperdeck & XLM around the hottub, lower deck area last June 2008. After a year I did notice black mildew spots only on the TwinFinish planks. We also have Azek trim which only had slight green film around the corner areas & inbetween the rails. I live in the Northeast & we just had 32 straight days of rain in May/June. I then noticed more black mildew only on the TwinFinishplanking. I ordered the Corte cleaner & was successful with it after only one application. I did use a pressure washer with a fan spray. The pressure washer was rated higher than the recommended max of 15psi. I just held it further away from the deck & it did fine. I was surprised to see the black mildew spots after thinking this decking would be relatively mainteneance free. We are planning to give it another cleaning w/Corteclean at the end of the season, then again when we open up the deck next spring. I will keep posting. I still think the Timbertech is a good material and wouldn't be afraid to use it.