Then maybe you need to look again at your methods. Stone, especially the softer varieties of sandstone and Limestone depending on where they are on the rock cycle have varying degrees of hardness on the mohs scale. Pressure washer guns, nozzles can damage the surface beyond repair if not used correctly. This is especially prevalent with cheap PW’s in the hands of novices. The problem is exacerbated when the same novice get their hands on Hypo and start mixing it and then applying it because they want a quick result. A bit like the novice bricklayer who thinks brick acid is the solution to any problem. The golden rule of cleaning ANY surface is firstly to identify it, assess the contaminants and then clean it in the most effective way without doing damage to the surface or the surrounding areas. If you were to burn a glass dish in an oven, you can either go mad and scrub it off with a Brillo pad or soak it overnight in some warm water with a bit of fairy and wipe it off with a microfibre cloth the next morning. The result is the same! Which would you rather do?