# What is a Scratch Coat?



## dnichols822 (Oct 12, 2008)

I've been told to apply a scratch coat of thin-set behind my cultured stone then put another layer of thin-set over the scatch coat and push the stones onto the thinset.  I don't understand the point of the scratch coat.  Do I let it dry for awhile before I put another layer on it.  If so how long normally?


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## Andy99 (Oct 12, 2008)

The scratch coat is to give your thinset a better bond to the sub straight.  If you are using a good quality latex motifed thinset over the proper substraight the scratch coat is not a necessity.


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## webbie (Oct 12, 2008)

I agree not needed over cement board, etc.
Thin set is amazingly sticky stuff.....does the job.
Over plywood and some other substrates this would be a idea.


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## wellbuilt home (Oct 12, 2008)

What are you working on ?     If its a in side sheet rock wall You need to use tar paper then a wire lath nailed to the studs in the wall.        Ive seen guy use thin set on WP sheet rock .     If you have a durarock or dead shield  wall  you could just spluge on the thin set over the wall then butter the stone and stick them on . I use a type s or n mortar  most of the time,  for every thing but sheet rock . If you use thin set on wire you need to cover the wire to build thickness  thin set  works best 1/4"   to  3/8" thick.  (scratch coat)  Mortar can go either way SC or not .


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## mranum (Oct 12, 2008)

When we did ours we followed the stone manufactures directions which was to put down the wire mesh over the top of the 1/2" OSB board,since it wasn't exposed to the elements no tar paper/moisture barrier was necessary.  Make sure the mesh has A LOT of nails in it, if it flexes put in more.  If it does flex the mortar will break eventually.  Wet the back of the stone then butter it up fairly heavy and work it into the mesh.  Make sure to work from the top down to avoid mortar dropping on the stone below.

We had some heavier stone pieces that wouldn't hold on their own so we supported them with some 3" screws from underneath them.  Worked well and just removed the screws the next day.


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## dnichols822 (Oct 12, 2008)

mranum said:
			
		

> When we did ours we followed the stone manufactures directions which was to put down the wire mesh over the top of the 1/2" OSB board,since it wasn't exposed to the elements no tar paper/moisture barrier was necessary.  Make sure the mesh has A LOT of nails in it, if it flexes put in more.  If it does flex the mortar will break eventually.  Wet the back of the stone then butter it up fairly heavy and work it into the mesh.  Make sure to work from the top down to avoid mortar dropping on the stone below.
> 
> We had some heavier stone pieces that wouldn't hold on their own so we supported them with some 3" screws from underneath them.  Worked well and just removed the screws the next day.



Interesting idea with the screws.  Do you know why you wet the stones first?  Also at what point should I smooth out the joints?  Right after installation or should I wait a while?  Thanks.


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## Elfin (Oct 12, 2008)

IMO, a lot of these steps are probably overkill. It depends on what you are working on (a hearth? floor pad?) and what kind of materials you are working with. Many of these steps might not be necessary at all. If you can tell us more about what you are applying your stone to, it would help. 

The purpose of a scratch coat is to give the substrate some "tooth" and to also smooth out any uneven areas or imperfections.  For example, if you're going over brick, you'd want to smooth the whole thing out first with a scratch coat.  But if your substrate surface is clean and smooth, or an ideal substrate like concrete board, there is absolutely no reason to take this extra step.  If your substrate is slick and you're worried about your thinset gripping it, you want to rough up this surface and apply a scratch coat. 

Yes, you let the scratch coat dry before you start setting your stone. Depending on how thick the scratch coat is, waiting overnight is certainly going to be adequate dry time. 

Let us know more so you don't end up taking a bunch of unnecessary steps.


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## mranum (Oct 12, 2008)

D-Nic said:
			
		

> mranum said:
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You wet the stone so it won't draw as much moisture from the mortar.  As far as the visible joint, give it overnight for the stone mortar to set then take a grout bag and mix up some slightly thinner mortar and squeeze it into the joints and work it smooth for whatever appearance you are looking for.  We smoothed it out then brushed it.  Works best to have 2 people doing this.

Here is a link to a video put out by Eldorado Stone if anyone is interested.  It explains the basic procedures.

http://www.eldoradostone.com/flashsite/


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## dnichols822 (Oct 12, 2008)

Elfin said:
			
		

> IMO, a lot of these steps are probably overkill. It depends on what you are working on (a hearth? floor pad?) and what kind of materials you are working with. Many of these steps might not be necessary at all. If you can tell us more about what you are applying your stone to, it would help.
> 
> The purpose of a scratch coat is to give the substrate some "tooth" and to also smooth out any uneven areas or imperfections.  For example, if you're going over brick, you'd want to smooth the whole thing out first with a scratch coat.  But if your substrate surface is clean and smooth, or an ideal substrate like concrete board, there is absolutely no reason to take this extra step.  If your substrate is slick and you're worried about your thinset gripping it, you want to rough up this surface and apply a scratch coat.
> 
> ...



I am applying the stones to durock cement board behind a wood stove.  Sounds like I don't need a scatch coat.


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## Elfin (Oct 13, 2008)

No, you most certainly don't. Just use a good modifed thinset, and you are all set!


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## wellbuilt home (Oct 13, 2008)

Yes you should use a scratch coat on the DR but don't let it dry . Do a spot about 4x4 and let it set for 15 mins then butter the stone and stick.


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