I really hate rust stains like that-it looks very deep and it will be tough to fully remove-However it is possible to get out a good amount.
Looks like water got behind the stone and an areas containing ferrous minerals reacted or is it possible some non stainless hardware was used to pin the slab at that point.
Can you consider replacing that piece??
I would open up the stone with an 70/80 grit electroplated or sandpaper discs.
Acidic salts seem to work best on rust-many acids will eat rust but also eat calcium as well.
I would then consider using Klenzstone number 2- it can be left on the stone as a poultice using paper towels or other mediums.
It isn't hard to get poultices to stick to vertical surfaces-like Carl said-use saran wrap if you don't use paper towels.
There are other chems tat can be used as well but Klenzstone will be easier to keep wet and get absorbed deeper into the face of the slab.


On Wed, Apr 6, 2016 at 9:35 AM, Randy Frye <rfrye6@gmail.com> wrote:
make a pocket out of plastic wrap taped to wall on three sides fill up with powder material that is already mixed with  liquid of choice then tape top. of coarse after all done with poulticing I would clean taped areas well with a citrus cleaner to get adhesive off. Good luck

On Apr 6, 2016, at 8:46 AM, Adam Bartos <info@restoreyourtiles.com> wrote:

I thought about poulticing  but how can you do it on vertical wall?

On Apr 6, 2016 7:35 AM, "Pat Staples" <cpstaples@msn.com> wrote:
I would try poulticing it out with the iron out if it does appear to be rust.



Happy Connecting. Sent from my Sprint Samsung Galaxy S® 5


-------- Original message --------
From: Randy Frye <rfrye6@gmail.com>
Date: 04/06/2016 8:26 AM (GMT-05:00)
To: sccpartners@stoneandtilepros.simplelists.com
Subject: Re: Stains on limestone - urgent

Maybe try sanding it out if it's just on the surface   

Best Regards,
Randy Frye, CEI Surface Pros

On Apr 5, 2016, at 10:38 PM, Adam Bartos <info@restoreyourtiles.com> wrote:

Last week we cleaned limestone walls and floors for the residential customer  using our truck mount. We came back yesterday to seal the floor and were surprised by the look of one slab, with one continues stain at the bottom.  It lookes like iron oxidation and it is only 1 slab out of 6. We tried iron out but that didn't help. 
Any idea why something like that happened?
What can I use or do to remove it?
I am going back there on Thursday and I need your  help very badly. I am puzzled and without any ideas. I am attaching picture for u to see it.
Thanks
Adam Bartos
BiO Tile

 
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Regards,
Stu Rosen
201-446-1200

"EVERYTHING MATTERS"