ANy idea

How to clean this it's a build up of rain water that's blackened the black soilders

 
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Can't jet wash as it distroys the render

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Hard to tell from this photo, id stay away from it personally but if I had to I'd go for sandpaper (long job) could try a chemical, but I can't tell if it's caused by rainwater leaking down or by rainwater mixing with whatever is in the mortar. If it were carpets I'd do a ph test on the stain to work out which chemical to use.


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It's a build up of rain water that hits a fibre glass roof and pours out the channel holes but then runs along the soilders and finds its way down to floor

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 softwash    but use   gel max    its sticks to it and is great stuff

but if ur new to it i say maybe  find someone to do it  as   you need a bit  of knowledge before messing with chems

Have you got a link where I can get it Pete interested in doing a test patch with it

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Is it the same as k rend.

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