Limestone ... agh! How to avoid colour change on sealing?!

[GrinAndBearIt]

Was due to get guys in to lay 60sq m of honed limestone on Monday ... when we discover that it is MUCH yellower than we wanted. Basically I think we looked at a tumbled version of this limestone, liked the colour, but went with honed (without checking sample) because we prefered cleaner, straighter grout lines and just assumed it would be same colour finish. Big mistake.

So ... in a big panic when we saw how yellowish this turned out to be. But when you look at the reverse of the stone, it's really nice and pale. So I guessed the yellowness must be from some sort of cheap sealant applied after the honing.

Sanded one of the tiles down using 40 grit sandpaper (I know, not exactly normal for stone etc.) ... but yes came up wonderfully pale! Rushed out to get some LTP Mattstone sealant (an impregnator, and reassuringly expensive at £17 per half litre thanks) ... but doh, back it goes to being yellow again.

These picture shows tile on left before sanding, and after sanding (ie. desired colour) on right. Does anyone therefore know a sealant that will produce the SMALLEST colour change possible? Desperate to buy something today ready for Monday ....!

 

[AdamCH]

You could try Lithofin MN Stainstop (not the plus which intensifies the colour) http://www.pureadhesion.co.uk/lithofin-mn-stain-stop-natural-stone-impregnator-sealer-1-litre.html

I've used it on Limestone and it didn't change the colour. It will have been a different Limestone though so no guarantee.

 

[GrinAndBearIt]

Thanks Adam.

Ok ... am now assured that nothing is done to this stone before it leaves the stone shop. It is basically just polishing (and honing) that creates the finished colour! So actually by sanding it, we weren't removing any seal or top layer as such, we were just roughing it up and as a result it looked paler.

So ... have now bought some MN Stain Stop (as lots of people have recommended) and also hired a floor orbiting polisher but with a 60 grit circular pad attached to do the roughing up and which hopefully will the do the job.

My God - ridiculous amount of stress on a Bank Holiday weekend ... but hopefully the answer is at hand!

 

[AdamCH]

60 Grit is quite rough for honing, I think a silicone carbide P120 and 220 and finish on 400 would be more like it. You can find out more on http://www.atsdiamondtools.co.uk/home

Is it possible that you're not just seeing surface oxidation and your sanding isn't just temporarily removing it?

 

[GrinAndBearIt]

When you say '60 grit is quite rough for honing' ... I don't quite understand:
at the moment I'm led to believe that it is the 'honing' that is causing this unwanted yellowish colour. By taking this 60 grit orbital machine to the stone, I'm hoping that we achieve a much lighter colour as a result of it ending up a bit rougher.

I attach a picture of the front and back of the stone tiles in question, straight as they arrived from the supplier. The one on the left is the top of the tile, and the one on the right is the reverse side. We infinitely prefer the colour of the 'unpolished' side!

Front (left) ... back (right)

Untitled

• GrinAndBearIt
• 24 May 2014
• 1

Hadn't thought about surface oxidation or even if such a thing exists in natural limestone ... but if it does, surely the back of the stone would have turned the same colour ... ?

 

[AdamCH]

Honing is just polishing the tile to a point less than a fine polish - some materials won't take a fine polish - many Limestones won't.

A high gloss would finish with a 1200 grit, honing would finish with something like a 400 grit. P60 is a rough grit for removing material, you wouldn't normally finish with this as the surface will appear very scratched.

No real reason why honing/polishing should change the colour much unless the dust is left on the tile.

 

[GrinAndBearIt]

Thanks Adam ... but can I just ask ... if honing doesn't change the colour very much, why then the very marked difference (as shown in the picture above) between the front and the back (and, for that matter, all 4 sides) of every single stone tile?

We are pretty much committed to the stone we have ordered. Work starts on Monday, so we have to make the best of what we have. But finished colour is much more important to us than texture - and if we can lose this yellowy orange patina through roughing it up, then I'm all for it. Actually, I think we would even prefer a slightly rougher & less polished finish ... if only for the sake of 3 dogs who will otherwise be slipping & sliding everywhere.

So I guess I'm only now just seeking reassurance that there isn't some factor we are overlooking, and also maybe to find the best approach to this kind of operation. Also just would love to know the physics of it - eg.what is actually happening when stone changes colour when polishing/honing?!

 

[AdamCH]

I'm not sure to be honest. You might try asking on a tiling forum with posters that lay lots of stone - there are lightening products that they might point you to. Or your tilers might be able to help you.

 

[frankmn]

Should contact a professional who has the expertise on this. mngranite