# Skim coat cracking



## jkfox624 (Jun 20, 2009)

Hung drywall in a basement job we were doing about a month ago. Had 2 sheets of cement board behind a wood stove that i taped into the drywall and skim coated. Looked fine when we left after sanding, HO primed it and its full of a bunch of spider web cracks. The board is solid, all taped seams are fine. Skim coat was just enough to cover its not like it was 1/4 inch thick. Done in 3 passes with green lid premix, dried in between coats and had 2 dry days before final sand. Any ideas?


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## Sir Mixalot (Jan 6, 2008)

JK, Sounds like you did everything right.:thumbsup:
Maybe the mud was thinned to much?:confused1:

-Paul


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## Big Shoe (Jun 16, 2008)

I assume you put the ''rough side'' out. Sounds kinda odd that it did that. We've skimmed over aot of cement board in bathroom areas, never a callback.

Maybe you flexed the cement board and it had cracks you did not see in it??? HO soaked it with too much primer??? Were the studs 16''o.c. Screws 8'' o.c. ???

Just skimmed a fireplace surround yesterday. Three coats 20min. Hope that does'nt crack!

And it's about time Durock's ''Next Gen'' really does cut easy!:clap:


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## jkfox624 (Jun 20, 2009)

Yeah rough side was out screwed 6-8" on center studs at 16. That was my first thought to was maybe the board was broke, gave it a good push inbetween studs and it was solid as a rock. I've always used the rule of thumb about an 1" of water in the bottom of my tray gets mixed in each bucket so i cant believe it was to thinned out.


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## BUTCHERMAN (Jan 19, 2008)

Four questions. Is the stove up and running? Did you prime board? Was it Durock or wonderboard? when you say green lid premix is that USG or Lafarge?

There are different applications depending on the answers. I will start by saying this. You should start with either Plaster weld or zinser bond coat. I would also suggest using Durabond or EZ sand mixed with either Kal coat smooth or diamond plaster for base coats. Also let me know if the paint peeled at all. I would be surprised if it didn't. I do know how you can solve this problem if you can answer these questions. There are different ways to go about this. I want you to use the best one considering the walls will go though temperature changes radically. You also need to be sure that your coat aren't peeling or came off with the paint roller.


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## Sir Mixalot (Jan 6, 2008)

JK,
It's very possible that all of the moisture in your skim coat was sucked out by the rough side of the Durarock. 

-Paul


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## jkfox624 (Jun 20, 2009)

The stove isn't installed yet. USG was the brand of mud. Bonding to the board doesn't seem to be the issue. Paint didnt peel any of it. I picked at the hairline cracks and its stuck. It seems like a shrinkage issues but i dont understand why. Maybe the board expanded slightly and the compound shrunk? I did coat the first coat with ez sand 45.


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## Bastien1337 (Dec 20, 2010)

could you coat again now that its sealed and the board cant take up much more moisture? just fill in the hairline cracks?


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## jkfox624 (Jun 20, 2009)

Bastien1337 said:


> could you coat again now that its sealed and the board cant take up much more moisture? just fill in the hairline cracks?


Yeah im gonna try that monday. Who knows the fine cracks coulda been there when we left but the dust had them filled. They primed, it pulled the dust out and highlighted them.


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## BUTCHERMAN (Jan 19, 2008)

I doubt it. The problem is when you used the easy sand you didn't accelerate it. Because of that it didn't take. If it was wonderboard and not durock it won't take because the finish is glossy and your products were wet too long. Even a straight 45 base needs help. But the thing that most likely did you in was a heavy coat of USG ready mix. It softened up the undercoat and separated it from the cement board which is meant to seal out moisture. The trick is accelerate drying time.

Here's what you do. If anything is loose scrap it down and use a bond coat primer. Since it's holding and not peeling you need to go over it with a mesh screen and set it like this. Use EZsand 45 but mix it loose (like banjo mud). Put some on you hawk or pan and throw it a handful of a veneer top coat plaster (Diamond or Kal coat smooth) and mix it in. This will accelerate the drying time. You want it to set up in about 15 minutes. The wall should set up before the EZsand it the bucket does. Then make another mix and trowel on a top coat to cover the mesh. Let it set up. Then one more time with another mix do a cross sweep on and off. Then you can use any ready mix compound you like and do another on and off in the opposite direction (so it drys quickly and gives you a good finish). This should solve your problem. Just remember the quicker it drys the better the chance of it taking. Also don't eliminate the plaster in the mix. You need it for bonding. You can also dry knife out laps with a veneer finish plaster very easy. Don't over work it or you will re-activate the EZ sand and it will peel. Sand it instead of over working it.

Good luck I hope what ever you do works out.

BTW where is Bradford? I'm from north east PA.


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## jkfox624 (Jun 20, 2009)

BUTCHERMAN said:


> I doubt it. The problem is when you used the easy sand you didn't accelerate it. Because of that it didn't take. If it was wonderboard and not durock it won't take because the finish is glossy and your products were wet too long. Even a straight 45 base needs help. But the thing that most likely did you in was a heavy coat of USG ready mix. It softened up the undercoat and separated it from the cement board which is meant to seal out moisture. The trick is accelerate drying time.
> 
> Here's what you do. If anything is loose scrap it down and use a bond coat primer. Since it's holding and not peeling you need to go over it with a mesh screen and set it like this. Use EZsand 45 but mix it loose (like banjo mud). Put some on you hawk or pan and throw it a handful of a veneer top coat plaster (Diamond or Kal coat smooth) and mix it in. This will accelerate the drying time. You want it to set up in about 15 minutes. The wall should set up before the EZsand it the bucket does. Then make another mix and trowel on a top coat to cover the mesh. Let it set up. Then one more time with another mix do a cross sweep on and off. Then you can use any ready mix compound you like and do another on and off in the opposite direction (so it drys quickly and gives you a good finish). This should solve your problem. Just remember the quicker it drys the better the chance of it taking. Also don't eliminate the plaster in the mix. You need it for bonding. You can also dry knife out laps with a veneer finish plaster very easy. Don't over work it or you will re-activate the EZ sand and it will peel. Sand it instead of over working it.
> 
> ...



Bradford is pretty much in the middle of nothing. We're about 75 miles east of lake erie right on the new york border.


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## BUTCHERMAN (Jan 19, 2008)

jkfox624 said:


> Bradford is pretty much in the middle of nothing. We're about 75 miles east of lake erie right on the new york border.


I'm in the Poconos, About 20 mins from Camel back mountain.


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## jkfox624 (Jun 20, 2009)

Straight from USG tech support, not that i doubted your opinion butcherman but i figured i might as well ask them. 

It is USG's recommendation that whenever a joint between a Durock Panel and a Sheetrock panel is going to be finished with joint compound that the Durock be sealed first and that Setting Type Compound be used. The make-up of the Durock will break down any Gypsum based compound that is applied to it unless the Durock has been sealed prior to the Compounds application. At this point there isn't anything that can be done except to remove the  Compound that was applied to the Durock. It will eventually begin to fall off of the Durock if it is not removed.


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## Metro M & L (Jun 3, 2009)

Op, you used a setting compound, the 45, I don't understand why you are going back to square one?


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## BUTCHERMAN (Jan 19, 2008)

jkfox624 said:


> Straight from USG tech support, not that i doubted your opinion butcherman but i figured i might as well ask them.
> 
> It is USG's recommendation that whenever a joint between a Durock Panel and a Sheetrock panel is going to be finished with joint compound that the Durock be sealed first and that Setting Type Compound be used. The make-up of the Durock will break down any Gypsum based compound that is applied to it unless the Durock has been sealed prior to the Compounds application. At this point there isn't anything that can be done except to remove the Compound that was applied to the Durock. It will eventually begin to fall off of the Durock if it is not removed.


You can still screen over it as long as the primer isn't peeling. Just remember the trick is to accelerate the drying time.


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## BUTCHERMAN (Jan 19, 2008)

Metro M & L said:


> Op, you used a setting compound, the 45, I don't understand why you are going back to square one?


 
Because 45 doesn't set up quick enough. Also if that first coat isn't on and off like painting it ends up sitting on top and that makes it peel. You need to kill the surface texture of the cement board and lock it up quickly to be effective. Then you floats can be heavier. I recomend multiple light coats to kill the surface. But a pro can work it right if he make it take before it starts to peel. Also not to do a heavy ready mix finish. On and off or it will soak into the surface and separate it.


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## Sir Mixalot (Jan 6, 2008)

jkfox624 said:


> Straight from USG tech support, not that i doubted your opinion butcherman but i figured i might as well ask them.
> 
> It is USG's recommendation that whenever a joint between a Durock Panel and a Sheetrock panel is going to be finished with joint compound that *the Durock be sealed first* and that Setting Type Compound be used. The make-up of the Durock will break down any Gypsum based compound that is applied to it unless the Durock has been sealed prior to the Compounds application. At this point there isn't anything that can be done except to remove the Compound that was applied to the Durock. It will eventually begin to fall off of the Durock if it is not removed.


Bingo!:thumbup: 


Sir Mixalot said:


> JK,
> It's very possible that all of the moisture in your skim coat was sucked out by the rough side of the Durarock.
> 
> -Paul


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## lrobertstoner (Mar 20, 2011)

i have done drywall since i was 16 years old now approaching 41 i have a tremendous off knowledge on the subject one thing i would like for ya to know about all quick set dry wall mud it takes for ever to dry totally it serfice dries in what ever time you want but if any amount more than a skim coat it takes a few days or longer the total dry time depends on moisture in the air and temperature those cracks can be easily fixed by applying a stiff coat of light weight all pourpose mud no water and skim out sand the light weight wont shrink as much if you are peeling mud then sand this back down to tape and start a new my suggestion to every one doing drywall use green lid mud for tapeing light blue lid for bed and skim for your money the best mud on the markett is magnum it costs little more but its well worth your $ yes this will delay drying time but if you are wanting perfection then you dont take steps to hurry the process up quick set mud is just a cheaters way and you will have more trouble with this then if you use some easy sand drywall mud such as the two i have offerd up i have never had the problem you are experiencing while useing a ready mixed mud and i have plenty of experience with durarock hope this helps you on future projects if you didnt bed that with a good bed coat or if your sheet rock was gapped this could be whats causing you fits try the lightwieght fix and see if that dont help this mud is perfect for a spot where a lot of mud is needed like bad but joints and corner bead not to good for angels easy to scratch


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## Muddauber (Nov 2, 2007)

Dang Ironbertstoner, I got a headache trying to read that!:blink:

Maybe leave out the STONER.:laughing:


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## BUTCHERMAN (Jan 19, 2008)

lrobertstoner said:


> i have done drywall since i was 16 years old now approaching 41 i have a tremendous off knowledge on the subject one thing i would like for ya to know about all quick set dry wall mud it takes for ever to dry totally it serfice dries in what ever time you want but if any amount more than a skim coat it takes a few days or longer the total dry time depends on moisture in the air and temperature those cracks can be easily fixed by applying a stiff coat of light weight all pourpose mud no water and skim out sand the light weight wont shrink as much if you are peeling mud then sand this back down to tape and start a new my suggestion to every one doing drywall use green lid mud for tapeing light blue lid for bed and skim for your money the best mud on the markett is magnum it costs little more but its well worth your $ yes this will delay drying time but if you are wanting perfection then you dont take steps to hurry the process up quick set mud is just a cheaters way and you will have more trouble with this then if you use some easy sand drywall mud such as the two i have offerd up i have never had the problem you are experiencing while useing a ready mixed mud and i have plenty of experience with durarock hope this helps you on future projects if you didnt bed that with a good bed coat or if your sheet rock was gapped this could be whats causing you fits try the lightwieght fix and see if that dont help this mud is perfect for a spot where a lot of mud is needed like bad but joints and corner bead not to good for angels easy to scratch


EZ sand does set up when you mix it with the diamond or kal coat. You can also dry knife it smooth as long as you don't over do it. I know what I'm talking about here. You are right about skim coats but you can't you ready mix over unprimed cement board. Your best bet is a durabond/veneer mix. You can also do an EZsand/veneer mix. But you always want to accelerate drying and keep the coats light. For you to call Ezsand the cheaters way means 1 of two things. You are an old dog that refuses to learn a new trick or you have a bias toward a compound you don't understand how to use. The veneer helps with the bonding and the acceleration takes away reaction time to crack of separate. There is no logic in your approach. Ready set compound just doesn't take. It's like saying you can use ready mix on wood lathe instead of a plaster base. It just doesn't hold up.


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