Afternoon, been asked if I can clean an original unpainted pebble dash wall. It has a lot of black organic matter, did a test patch and the mortar just sucked the solution in and the results were probably 30% where it needs to be. Anyone here have experience on these? I was thinking maybe a weak solution to saturate the substrate, then follow up with a stronger mix for the surface? Its a bit cold here so waiting on a warmer day to try out different ratios etc?