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Window cills gone green

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Hi, started our new build around 9 months ago, floors were put in last week but I've noticed the window cills are turning green outside, damp weather ehh?! Can you protect concrete cills from discolouring and algae? 


Has anyone any tips? Or know and used any products to weather proof concrete cills? 

Edited by NIreland
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Copper sulphate solution works pretty well, and tends to be persistent enough on concrete to stop regrowth for a couple of years.  Bleach or Jeyes fluid will kill it and clean it up, but the latter smells awful.


I think the best approach would be to clean them up with diluted bleach and a scrubbing brush, then rinse them off and allow the concrete to dry out.  Mix up a strong solution of copper sulphate (buy some copper sulphate pentahydrate crystals and gradually dissolve them in water until no more will dissolve).  Brush or spray this solution on to the concrete, and don't worry about the bright blue colour, it isn't permanent.


Allow the cills to dry out and the residual copper sulphate in the concrete will inhibit algae and moss from growing for a fair time.  The rain will eventually wash the copper sulphate out of the concrete, maybe after two or three years, but just apply some more and you'll have protection again.  This also works on concrete paths that are prone to going green, or concrete paving slabs.  Use with caution on stone, though, as some types of porous natural stone may stain - test a hidden area first to be sure.


You can buy copper sulphate crystals on ebay cheaply enough.

Edited by JSHarris
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