stuart noble wrote:

On 10/09/2010 16:06, Malcolm wrote:
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
Take care with oxalic acid - it is poisonous. It is the chemical
whic
makes rhubarb leaves dangerous.

Malcolm

The first thing I'd try is neutralising with washing soda. Not sure
why
the acid would have produced a brown colour but the salts of said
acid
may wash off more easily.

Iron in the sandstone? Maybe we are dealing with rust.

Edgar