Help with Black Spots after grouting

[liamb2001]

Hi everyone,

Could really do with some advice.

First off i want to say a huge thankyou for paving expert, it was this website that proved to us our first builder had not used a full mortar bed, claiming dot and dab was fine for indian stone. This saved us a lot of money and stress! Ive been a huge advocate of reading how paving should be done from this website ever since!

We have pointed a few areas of our paving with easy joint, without problems until recently but on saturday we pointed it with the same stuff, kept it wet and brushed in keeping it moving as usual but as it took a little longer than we were expecting it started to get dark and we clearly must have left too much product on a handful of the stones anmd its left some nasty black spots (see pictures)

https://prnt.sc/7u5zOzCFzpNK
https://prnt.sc/CwPXTQkN_mXM

We have had these appear on small areas before and used diliuted brick acid and a jetwash and theyve faded but with these handful of stones its just not coming out and was wondering if anyone could advise what we could possibly do to fix them they look awfu!

We have been told to try grout residue removal, grease removal, and patio cleaner but we are aware with inidan sandstone its porous and dont want to keep hitting it with different checmicals which not only could potentally damage the stones but if they dont work will be costly.

Any help or advice on specific products or methods would be greatly appreciated.

[liamb2001]

sorry should have said The stones are kandla grey indian sandstone 25mm.