Had a different job yesterday: lady with limewashed walls that had gone green wanted them cleaned off.I had done them before with pure with limited success. She did the research and came up with two cans of Dulux Weathersheild anti fungus solution. You just brush it on, leave for twenty four hours and rinse off. I used the van pump and microbore to apply it with a vikan sill brush - dead easy. Going back later to rinse down. It might take all the effort out of scrubbing to kill the algae chemically. She supplied the stuff so i don't know the cost of it. Might be a market for this?