I don't know much on this subject, but I was talking to one of my cusomers, who has just bought a low pressure sand blasting machine, and it cost a small fortune, £17,000
she has taken on a lot of work for a grave stone masons, the reason for this, was they use high power pressure washers, and this used to damage a lot of the stone, I think it may have been hamstone she was talking about, this is very soft stone, all's i'm trying to say is stear clear of any soft paving stone with high power pressure washers. but there is a lot of work out there for pressure washing.