Revisions log for Answer #445

Last year we switched from pressure washing with just water to brushing after pretreatment with caustic soda in the courtyard (concrete block paving), water comes from a hose with a spray nozzle. So there is less pressure. In my opinion, the high pressure cleaner has a double effect: the high pressure only sprays a little more of what was previously sprayed deeper into the pores. The caustic soda reaction loosens the dirt better than a rough attack with a high pressure machine.