Removing rust from stone can be extremely difficult but with the correct products it is easy.
Look at the pictures below and you will see that top one is a picture of a sandstone wall that has been stained over the years with rust that has washed off from the railings that used to be attached to the wall.
This wall has been like this for over 13 years and it would seem almost impossible to remove the rust marks. When the delivery driver delivered the chemical that I used to clean the wall he said that I had no chance of removing the rust stains.
This wall is badly stained with rust, this has been cleaned with a power washer but it had little effect
NET-TROL is the product I used to clean the wall above
You can also use oxalic acid but in the past I have tried both and NET-TROL has always given good results
Now before you do anything make sure that you are covered up, so a pair of overalls is a good idea. Goggles and rubber gloves should also be worn as the Net-trol is acidic. Net-Trol is plant and pet safe and so I am probably being over cautious here! Apply the Net-Trol to the stone using a brush and leave it for 20-30 minutes.
After the 20-30 minutes has passed brush the stone using a stiff brush ( not metal ). Here I have used a deck scrub.