Revisions log for question
How do I clean tannic acid stains on my patio?

We are currently building a patio roof. To protect the sidewalk from being stained by the painted beams, I put some wooden slats under them so they wouldn't lie directly on it.

Now for the positive: the paint didn't stain. The negative: one of the slats was oak and left an ugly streak on the sidewalk after it rained. There was no way I could explain it and I started googling... and here it is: the wood reacts with the lye of the concrete sidewalk and releases tannic acid, which leaves a rust-like residue. 

Is there any way I can clean it again? Has anyone had any experience with this issue? Otherwise I will have to replace about 25 stones...

• stains on sidewalk