I have an external retaining wall - concrete blocks which has been clad with thick sandstone tiles and sealed, some 15 years ago. The wall is now dirty and has what looks like black mould. I soaked the wall, then scrubbed it with brick and stone degreaser/cleaner, and rinshed it which has helped but it still shows stain spots. and below after scrubbing
I would have thought a specialty paver cleaner would do the job. You could also try more natural methods like white vinegar and tea tree oil. I imagine you can't blast the wall with a high pressure cleaner as the slate might chip off?
oops, sorry I meant sandstone not slate. Unfortunately vinegar and other acid based cleaners will damage the sandstone.
Thanks @Kermit - I couldn't find the video link, but did google Oxy-Klenza on Spec-Net.com.au. I'll try pressure washing, and chlorine and let you know which works.
The link was on the words "this video" on Kermit's post above.
Good luck with the cleaning.
Chlorine will definately do the job.Dissolve about one quarter of a small cup of granular pool chlorine in a bucket of water,then paint on with old paint brush,cover wall thoroughly, leave for an hour or so then hose off.Make sure you wear rubber gloves ,eye protection and old clothes.Avoid breathing the fumes.The run off will not harm the grass