In how much of a hurry are you? I had some Indian sandstone laid
(always a mistake: if you want a good job done, do it yourself) by a
builder who managed to tread mortar slurry all over the surface, so I
cleaned it off with brick acid, and had just the result you describe.
Two years on, the excess redness has pretty much weathered away.
Thanks for that Richard; it's nice to know it will weather eventually. I
tried the pressure washer this evening and it seems to have dulled it
down a little. Parents' pressure washer is a bit crap tho'. Will try
with a more powerful PW if the Oxalic acid has no effect.
Just to be clear for the pedants, it is in fact limestone, but it is
(apparently) sometimes called Indian sandstone.
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!
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
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
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.
On Wed, 22 Jun 2016 08:35:02 -0700 (PDT), email@example.com
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.
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 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 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
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