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...
We had this on our beige sidewalk too. Our child was given an oak sandbox shortly after moving in, and we put it on the patio before the final spot was chosen and the hole dug. Then the rain smeared tannic acid all over the sidewalk in a square shape.
The good news is that it will pass. The bad news is that it takes a long time. With a scrub and water, it got a little better, but it was still very visible. I don't remember if we used chemicals at this time. But time, sun and rain helped.