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 like magic.

150g/litre in very hot water in a bucket took the stains out in about 10
minutes. I made up 2 litres and by the time I'd used about 1.5 litres,
the water had cooled and there was a load of undissolved stuff in the
bucket. So I added another pint of boiling water and finished off.

100g/litre would probably have been just as good if a little slower.

It leaves little sparkly crystals as it dries, so you need to rinse it
off thoroughly whilst lightly scrubbing with a deck brush.

Note that they (the parents) have dark grey concrete blocks as a border
and these ended up being much lighter afterwards. A further rinse and
scrub (of the edging stones) improved the situation no end and I'm sure
they will weather quite quickly.

Ironically, a further application of brick acid cleaned a sample block
up nicely, I just had to be careful not to get any acid on the limestone!