I eventually finished laying some Indian sandstone flags (mint)at the front of my house in September. Your website and forum were invaluable in assisting me in various issues I had throughout the project. The flags are layed on top of previously layed concrete slabs with about 2 inches of sharp sand. To protect the flags I decided to use some Thompsons quick drying patio seal. I applied 2 layers and was reasonably happy with the result at first despite the 'wet look' and slight slippery surface.
However after a few weeks I noticed one of the cut stones was turning from a light green colour to 'black'. This stone is now about 75% black with the exception of the outer edges of the flag. As there was only one small cut stone with this effect I assumed it must have been something to do with this particular stone and thought nothing more of it. However after looking at the flags tonight there are several others now with spots of black appearing which I suspect is going to get much worse. The black looks like moulding of the sealer and is on the surface of the flags. It can not be removed with a pressure washer.
Have you ever heard of this and what I might be advised to do to try to rectify the situation. I am planning to contact Thompsons as it must have something to do with their product. I followed the instructions on the tin and it is supposed to be a clear barrier.
I can e-mail you photos if you like.
Thanks
Sandstone flags - Thompsons quick drying patio seal
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Tony McC
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