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I'm guessing the staining is in the surface texture of the stone.
How long did you leave the masonry cleaner on for?
You might need extended contact time to get into every pore of the stone.
You'll lose nothing by applying some more tonight and leaving it overnight.
mespilus-
The staining is / was mostly water I think but there is an unevenness to the colour as the stone is quite porous which is why I haven’t used bleach or a pressure washer. Here’s a better picture.
I’ll try another go with the chemical cleaner tomorrow, thanks. Ideally I’d like to restore some of the colour to the stone before I seal it. Has anyone any experience of using colour restoring like this?
freddo
Hey hive mind. One of the many jobs this weekend is to clean the flagstone steps to my house. I’ve removed most of the moss and mould off using Zep masonry cleaner but some small areas of black mould remain. I’ve tried vinegar and also a 2.5% solution of sodium hypochlorite, neither of which have worked. Anyone got any suggestions?