Forum: Flags and Slabs
Topic: Indian stone flags (kandla grey) staining
started by: Exa

Posted by Exa on 03 Oct. 2019,20:45
Hello all,

I'm looking for some help with some recently laid Indian stone, Kandla grey flags, I see they are a popular choice and I found a similar problem on these forums!

The patio is 3 months old at this point, some of the flags have a brown tint, two of them specifically are really bad. I've read it's a mixture of salts/minerals, which doesn't sound a cause for concern??

I've tried to clean them, even today give one of them a good scrub with warm soapy water and a hard brush. The foam from the soap did turn a little brown but it hardly made a difference.

I've attached some pictures, it had just started to rain when I took them... Some of the other flags are turning a bit, you'll it slightly in the last picture, it's the same as the others just not as bold (yet?)

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I haven't yet tried a pressure washer, I don't have one but I will be able to borrow one without issue. Or do I need some specialist cleaner?

I also have a question about sealing, I chose not to have the patio sealed initially, due to cost and it's something I could do myself later down the line, they weren't acid cleaned or anything like that either. I've read that Indian stone flags should not be sealed, and I've read the opposite.

Any advice/products etc will be appreciated, I just want the patio looking spot on again!

Exa

Posted by Tony McC on 04 Oct. 2019,09:36
It's natural iron colouration/mineralization within the stone, and there's nothing you can do about it.

There is natural variation in most stones and, in the case of Kandla Grey, there are some seams or beds that have this higher density of iron minerals. Once exposed to the damp of the British or Irish climate, the iron becomes a rust, which then discolours or 'stains' the stone.

It doesn't affect every piece of stone. It may affact an entire flagstone, or it can appear as a patch or a small stain, and it's largely unpredictable.....but not entirely unpredictable.

What we see is that the top brands exhibit far fewer instances of this phenomenon than the bargain brands, which indicates that, somewhere along the supply chain, some gadgie with a keen eye is hiocking out the suspect pieces to create a 'first class' stock with few, if any, suspects, while the stocks intended for those buyers who are concerned only with price will either not be scanned and selected, and may even be 'bumped up' with the suspect stone taken from the 'first pick'.

While a good quality rust remover will shift the staining for a while, it more than likely *will* return. So, that leaves you with two options: either learn to live with the natural variation of the stone and accept it for what it is, or get the naff pieces replaced.

Posted by Tony McC on 04 Oct. 2019,09:36
It's natural iron colouration/mineralization within the stone, and there's nothing you can do about it.

There is natural variation in most stones and, in the case of Kandla Grey, there are some seams or beds that have this higher density of iron minerals. Once exposed to the damp of the British or Irish climate, the iron becomes a rust, which then discolours or 'stains' the stone.

It doesn't affect every piece of stone. It may affact an entire flagstone, or it can appear as a patch or a small stain, and it's largely unpredictable.....but not entirely unpredictable.

What we see is that the top brands exhibit far fewer instances of this phenomenon than the bargain brands, which indicates that, somewhere along the supply chain, some gadgie with a keen eye is hiocking out the suspect pieces to create a 'first class' stock with few, if any, suspects, while the stocks intended for those buyers who are concerned only with price will either not be scanned and selected, and may even be 'bumped up' with the suspect stone taken from the 'first pick'.

While a good quality rust remover will shift the staining for a while, it more than likely *will* return. So, that leaves you with two options: either learn to live with the natural variation of the stone and accept it for what it is, or get the naff pieces replaced.

Posted by Exa on 04 Oct. 2019,17:34
Thanks Tony

What rust remover would you recommend?

In terms of sealing, could that help? (Eg. Remove rust then seal it to protect it), I'm not suggesting that's s fix as obviously the seal will wear away over time too

Posted by Exa on 04 Oct. 2019,22:52
https://www.smartseal.co.uk/sealers/natural-stone-sealer/sandstone-natural-stone-sealer-25-litre.htm

Can anyone recommend this?

Posted by R&A Pressure Washing on 06 Oct. 2019,15:41
Quote (Exa @ 04 Oct. 2019,17:34)
Thanks Tony

What rust remover would you recommend?

In terms of sealing, could that help? (Eg. Remove rust then seal it to protect it), I'm not suggesting that's s fix as obviously the seal will wear away over time too

It can be very hit & miss doing "rust" or iron stain removals, you could try Lithofins Rust-Ex as a starting point see how that goes, after that you'd be getting into specialist type chemicals and equipment etc. Sealing isn't going to help, the stain is with-in the stone so point-less sealing it.
Posted by R&A Pressure Washing on 06 Oct. 2019,15:43
Quote (Exa @ 04 Oct. 2019,22:52)
https://www.smartseal.co.uk/sealers/natural-stone-sealer/sandstone-natural-stone-sealer-25-litre.htm

Can anyone recommend this?

No I wouldn't recommend their products.
Posted by Tony McC on 07 Oct. 2019,09:18
I'd side with Roger - SmartSeal sealants tend to be 'jack-of-all-trades' products intended to be daubed and slathered over practically any stone or concrete surface, but the results are not always what everyone wants. They're fine if you like the heavy varnish, glossy, obviously sealed look, but for anything more subtle, you need to look elsewhere....and spend a few quid more!

Rust-Ex is usually reliable with simple rust stains, as is RCI from Weiss, but rust stains are so variable in nature that you can never be certain of being able to shift them
without repeated treatments....and there's a risk that repeated treatments, while shifting the rust, may also be damaging the substrate.

Posted by Exa on 07 Oct. 2019,21:57
Thanks both, I will give Rust-Ex try

You're right, I'm not after obviously sealed, just a matte/subtle finish, while I appreciate the iron in the stone, it's only come out when being exposed to the wet etc?

I thought using a good cleaner and sealing afterwards (and sealing the problem stones heavily), that would keep the weather off for a while? Maybe something to look at in spring next year

Posted by Tony McC on 08 Oct. 2019,12:02
Yep - definitely past the sealing season now. It's persisting down outside as I type!
Posted by Tony McC on 08 Oct. 2019,12:02
Yep - definitely past the sealing season now. It's persisting down outside as I type!
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