I was thinking pump up sprayer and straight bleach and it’s going to have to be pretty low pressure cause the stucco is old ideas??
Definitely low pressure. We use our roof pump system when we clean it.
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You don’t need to go anywhere near that strong. if you have pic can give you better idea. Would start with slightly stronger HW mix, say 1.5-2% and go from there. If it’s painted be careful with your mix and do a test spot.
I don’t think many downstream injectors are capable of putting out that strong of a solution. Most DSI’s are 10:1. With straight 12.5%, that’s 1.25% SH reaching the siding.
Whenever the need arises for me to put a stronger mix on something, I think I’ll be more inclined to invest in a small softwash pump than go to the dark side and get an xjet… lol 
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Don’t overthink it - Its just stucco.
Downstream your 12.5%, let dwell, rinse with soft wash tip or ball valve. May need two applications for real heavy mildew and mold. Never use high pressure on stucco, will leave “zoro” marks, even on not so old paint.
Would you dilute the bucket of 12.5 or use a 5 gal bucket of straight 12.5?
