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Paul8434 10-23-2013 08:42 PM

Scratch Coat Questions
This pertains to applying stone veneer to the fireplace surround sheathed in concrete board.

When making up the scratch coat is there a time limit with respect to how fast should the stone veneer should be laid before I lose any structural value from the mortar to scratch coat adhesion.

Because the stone lay up will take me a few weeks I was thinking that the scratch coat drying completely may have some adverse affects.


JulieMor 10-26-2013 08:13 AM

I'm in the middle of doing the same but outside. Everything I have read from the stone and the mortar manufacturer say the scratch coat should be allowed to set for at least 24 hours. There was no limit to how long it can sit after that before setting the stone. Some suggest using a spray bottle and wetting the surface of both the stone and scratch coat before applying the mud and setting the stone.

stadry 10-27-2013 06:22 AM

wetting an existing surface helps prevent existing dry surface from sucking the needed water out of the freshly applied ' stuff,,, were that to happen, it would weaken the new 'stuff ',,, there are specific bonding mtls but few actually use 'em,,, the condition you want is called ' surface saturated dry ' - the surface's wet no standing water's evident... obviously MUCH easier on vertical & overhead surfaces.

we usually use a water hose + leaf blowers for flat work :yes:

ddawg16 10-27-2013 07:05 AM

I'm not the expert....but have done it.....after a couple of pieces it becomes obvious what is the right amount of water.

isitreallyconc covers it pretty good....

Me? I use a hand spray bottle. I get the spot I'm about to cover wet...but not so wet that it runs down. I find that a moderate first spray before you start mixing up your mortar....then followed up with a as needed spray seems to work well....The color is a good the surface starts to dry, it gets lighter....

The key is that you don't want it so wet water runs down. What is really cool is that you can experiment with the water seeing how much it takes before you do the actual mortar....if you put too much...dab it off with a towel....or just let it air dry....

Don't be afraid of will get the hang of it quickly....

One thing you don't want to be lazy with....scraping out the excess it before it starts to set.....since us DIY guys take for ever to do these kind of jobs....clean as you will be glad later.

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