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You may be interested in this product. it is a coating that can be applied that uses nanotechnology, when applied on a clean surface dirt, moss, mold can not get a hold and simply washes off with rain http://www.nanovations.com.au/Concrete.htm watch the "product performance video" half way down the page as a demo of how nothing sticks to it.

Although this is an Australian company they export 98% of what the manufacture to over 30 countries around the world and are one of the World leaders in this technology, and it is very economical.