No. my logic is that a surface coating of lichen is neither a stain, nor permanent.
OTOH if it wasn't sandstone but limestone, all bets are indeed off. But limestone fizzes like crazy when you pop acid on it.
: > : > : > : > : sandstone is not damaged by acid. : > : >
: > : > So you are saying that the OP is lying?... : > : >
: > : >
: > : He never said it was damaged. Merely that it tuned red. : > Painting it with : > : red paint turns it red as well. : > : : >
: > It also damages the surface, especially if it is meant to be - : > err - sandstone coloured - and the paint won't come off. : >
: > By your 'logic' acid doesn't damage anything, assuming that the : > application was intentional, duh! :~( : >
: >
: No. my logic is that a surface coating of lichen is neither a stain, nor : permanent.
Whhoooossshhhh.........
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!
Result!
There seems something slightly counter-intuitive about correcting the effects of one acid by application of another.
Well, I haven't even begun to think about the chemistry. The tan colour just faded away in a few minutes without any fuss or fizzing. As you say, Result!
It was always recommended as a bleach for wood, though I never figured out how it would work.
The OP has observed otherwise, and Cormaic's pages at pavingexpert.com also describe the same problem. The various metal salts in the sandstone can react with HCl to produce a different colour.
Note: I am pretty sure the OP is describing immediate staining, not a lichen buildup.
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.
I know this is about 6 years ago now, and the chances of anyone seeing this are slim, but did this eventually work for you? I am in a similar situation.
I would suggest that after you've applied the oxalic acid (if you're going down that road), flush it well and repeatedly and rinse away the flushings, otherwise the iron, which is the cause of the staining, will just return, if a little spread out and lighter, as the OP found.
If its iron, I wonder if a magnet might be useful? Just a thought. Brian
Dave, thanks for posting this. I too have a sandstone walkway and steps tha t I got the acid stains on. I was trying to wash some excess mortar off the steps and walkway using a pretty diluted mix of muriatic acid and water. A few days later nasty orange and yellow streaks and spots showed up where I believe the acid sat too long or didn't get washed off well. After power washing, bleaching, and other methods I thought I was going to have the remove the yellowed stones. Today I purchased the oxalic acid in a crystal powder form from a local paint store. They are selling it as a dec k brightener. I put some hot water in a bucket and dumped some oxalic acid in. I'd guess I put in about two to three cups of powder for about 2 gallon s of water. I did not boil it, as it seemed to dissolve fine with the hot t ap water I used. I did stir it up. I then poured it over the areas and work ed it in with a stiff broom for about 10-15 minutes. I could see most of th e stains dissolve about 10 minutes into it and some spots took longer. The water/oxalic acid mix would yellow as it lifted the rust/muriatic acid stai ns off. I then power washed the entire area really well. The yellow stained stones looked as good as new! I'm guessing like Dave posted that they may return and it will need another washing. I plan to seal the patio once I fe el like it is resolved fully in order to prevent anything else like fertili zer from affecting it. Thanks again for this post.
replying to Dave Osborne, Craig S wrote: Hi Dave, I've just finished laying 46m2 and cleaned off the cement with what I thought was limestone/sandstone friendly cleaner. This was not the case and I've end up with brown stains, almost a rust colour. I've ordered some oxalic acid and plan to carry out the same as you did on Monday. Can you confirm if your parents slabs are still ok and the stains have not come back please. I'm gutted as so much hard work has gone into this! Regards Craig
replying to aaronschoen, KB wrote: Out of interest did the staining come back. I'm also in a similar situation. I followed your steps above and with in hours the staining is back, as you say less aggressive and more spread out. I'd be interested to know how many washes it took to remove it completely.
replying to Dave Osborne, Shanna wrote: Did the oxalic acid work
Do you think after a mere 7 years the OP will remember?
This might assist you with posting to a newsgroup, albeit through a website:
- If you are sending a reply to a message or a posting be sure you summarize the original at the top of the message, or include just enough text of the original to give a context.
replying to KB, Cal wrote: hi, I too am in this mess, first wash of the limestone with Oxalic Acid removed the stains but unfortunately they are back after a few weeks, as mentioned in other responses they are less intense BUT still there. I am now on the second wash with the Oxalic Acid so hopefully this will reduce it further, otherwise I am going to keep the process going and hope that it helps
hi did the stains go ?
Is this only a year old? Must be a record from home owners consite. Brian
York stone is sandstone
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