Its an interesting question whether or not this is better or worse than brushing the green off for example... A Q I can't answer!
What is it that breaks down the silica? Is it the pH difference? Is it a corrosive property of the bleach? If it's the pH difference, will chalk not have a similar effect on sandstones as it dissolves into rainwater and works its way into the porous rock?
Personally - as someone who climbs quite a lot on sandstone, I think using bleach is being lazy. Dry weather and a soft brush has always worked for me.
OK - spoke to my colleague. A geologist with a strong chemistry background (poor thing...)DON'T USE BLEACH ON SANDSTONE/GRITSTONEIt breaks down the silica in the cement holding the grains together. His words were "if you put bleach on sandstone, then let it wash off and repeat - eventually you'll be left with a pile of sand". Its not going to cause it to fall down, but regular use of bleach will eventually break it down causing it to crumble.
Vinegar on sandstone/gritstone would be fine - but bad for limestone!!
Quality reply Tom, thanks. Sounds like this kind of information should be better known.Elsewhere on the Friends of Pex FB page people are talking about a 10% solution and that venue would at least be subject to rain. Overhanging algae-riddled buttresses less so. Bleachers beware.