D-Limonene (orange oil) is expensive and not cost effective for house washing.
Sometimes the stain is in the paint and becomes permanent.
Sometimes the stains are in the paint, it is painted over then the mold, mildew and algae comes through the new paint and brushing it takes off the new paint (new paint never had a good surface to stick to on top of the contaminants) and then you still have the old paint, what is in the old paint and a mess of it all.
When talking to customers I let them know when I see situations like this or others involving gutters I let them know about it and let them decide if they want me to brush the gutters (which I charge more for and tell them).
Have you tried F-13 stronger? If not call Bob and get his opinion on how strong. I have used this and it has worked good for me.