i have finished decorating but i have one interior stone wall, any recommendation how to clean off the white (slightly chalky) surface muck to get back to the nice slate grey original colour.
I would leave the lichen where it is, it looks right in place.
If you were sure you wanted to get it off, then I would put some old painting or cloth sheets on the floor, but crumpled up right close to the wall, then set too with a little washing up liquid and warm water, using a couple of those green scrubby pads you clean pots with, the flat ones, not the one with the sponge attached. Try a couple and see what happens.
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