I can highly recommend this product for Indian sandstone that has become impregnated by a ten year build up of black mould/lichen which a pressure washer couldn’t touch. I chose to go the work on a drizzly day so the stones remained damp and used a sprayer to apply the solution non-diluted. I gave the stones a good scrub with hand brush to work the solution into the surface, then left for 15-20 minutes while sorting out another area. I then pressure washed and bingo! What a result! My ten year old, shabby looking patio looks better than new! Some stubborn areas needed additional spot treatment and the job was a significant time commitment for the 7mx7m but well with it. I used about 12-13 litres for this area. See before and after photos