What are these black spots? Pre treated twice with house wash mix then applied 5% through a sprayer and it won’t come off.
Are those just spots the organic debris damaged?
My first thought would be tannins, but hitting it with that high percentage should have removed them if it was. I hate to be THAT guy, but your SH isn’t old is it?
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Possibly lichens. Stubborn little buggers.
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Are they under an oak tree?
Possibility, I don’t quite remember. I think the SH is fine, it killed everything else just not that. I even tried with pressure, nothing.
Is that limestone?
I’ve had two instances where I had to go full strength after repeated attempts with a lower % and pressure. Full strength finally did it in both situations.
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Pour some 12.5% just on one of the larger spots and leave it for 5-10 minutes and see what happens
If straight SH don’t take care of it, I’d move on. Much success.
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Turbo nozzle?
dwell time is longer for those
