[Evans, Marv]

I found (at least with decks I've had) it helps to wash them down once in a while. I used a mist spray followed with something like car washing soap (gentle) with a soft brush, finished with a gentle wash off of water. Particulate matter settles on the planks and adheres to them degrading the finish. If left long enough a black, sort of sooty looking layer will form which gets harder to remove the longer it's left on. Keeping them clean seems to add life to any type of decking material.