what is good to clean black algae looking staining from sandstone? I tried soapy water, made little impact. I tried a water and bleach solution, again little impact apart from removing the moss/lichens growing on them.
If I can get them clean, is it advisable to use some some of sealer to prevent it happening for a while?
If I can get them clean, is it advisable to use some some of sealer to prevent it happening for a while?
