Dave Osborne wrote:
The parents have had a patio laid in Indian sandstone/limestone. Dad
decided to try getting a few mortar stains off with acid-based patio
cleaner. After a week or so, the patio has turned orangey-brown where
the acid has been.

According to paving expert, the acid staining is permanent. :-(

Anybody know if you can get rid of acid staining? Oxalic acid-based
scouring powder seems to be an option, but paving expert cites oxalic
acid as "unproven". Is it worth a go?

Cheers,
DaveyOz


Well, the Oxalic acid worked brilliantly for a day or three, but the
brown stains have reappeared over the course of two or three weeks. The
stains seem to be more "spread about" and less intense than they were
before.

We just re-treated with Oxalic again this weekend and again, the brown
stains all disappeared within a few minutes. I imagine that the stains
will come back, hopefully less intense again. This could be a longish
term project to gradually get rid of the stains.