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I have the fiberglass roof as you do. I just wash it with soap and water and use a long handle scrub brush. On my roof the black stuff is just dirt embedded in the pebbled surface. The brush gets most of it off. If I wanted to get it all off I think it would need to be scrubbed with a firm bristle hand brush.
I may try it one of these years but it would be a lot of work. I'm not quite ready to work that hard at it yet.
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