Stains After Pressure Wash

Hello, I just finished a concrete driveway pressure washing job and it looked good when I left. But then, the customer sent me some photos of their driveway with these black stains or spots on it. I don’t know if its organics or what it is. I pre treated and post treated the driveway (never washed the post treat off). I noticed it on another driveway I did as well and when I went back over it the spots persisted. If anyone has any advice I would greatly appreciate it, thank you!

Are you referring to the million little spots or the 2-3 large areas?

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Probably magnesium, doesn’t come off.

The 2-3 large ones. Not sure what’s causing it. Any advice would be greatly appreciated :slight_smile: