Patching plaster top coat over rock lathe


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Old 09-25-22, 09:18 AM
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Patching plaster top coat over rock lathe

Hello, I am looking for guidance on how to patch this plaster wall. The top coat of plaster separated from the rock lath. I have peeled back the edges to the point that the plaster is firmly attached to the lath.

I am wondering what materials to use, looking for the easiest approach - this is inside of a closet, so it doesn't have to look perfect. And I am a novice, so I need to keep it simple.

I have found lots of conflicting information online. Do I need to apply a bonding agent (and if so, which one) first? Could I apply Plaster Magic conditioner instead as step 1? Next, could I use joint compound? What type? Do I need to use any kind of tape to attach the edges?

Thank you so much for any guidance!


 
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Old 09-25-22, 11:51 AM
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This sure looks more like poured concrete than gypsum lath. USG’s RockLath is one brand of gypsum lath as SheetRock is USG’s brand of gypsum drywall. Anyway, how thick is the plaster? The pieces on the floor look like about 3/8”. It would be nice to know what the base coat is. It might be Portland cement plaster or it could be gypsum plaster. Here is how I think you can fix this if I am right about this being poured concrete and thinking it is about 3/8”. Get the loose stuff off. Attack it with a floor scraper or flat chisel. What is bonded will not be harmed, at least not much, and what is not bonded should come off. Then wire brush the exposed concrete. If there is a way to handle the water I would advise you to wash it with a 50/50 vinegar solution and rinse it but you might not have a way to drain the water. Or even a <10% Muriatic acid / water solution if you can rinse this well and get rid of the water. Let it dry. WeldCrete is a good bonding agent but I don’t think you can buy it in less than gallons so use a bonding agent like Quick Crete’s bonding adhesive NOT Concrete fortifier. You can buy the QuickCrete bonding agent in quarts. Now someone might be wondering about concrete fortifier. It adds strength and stickiness and reduces water requirement it is not a bonding agent. Well, I digress. Do this then mix up some mortar mix. It is easier to use mortar mix than make to make your own with Portland, sand, lime etc. and you don’t need as much as a whole bag of Portland will make. Spread this on and flatten it out with a straight board and make it nice and flat, not smooth. Let is set a day or more. For the finish use joint compound. Put some of your bonding agent in the joint compound or some latex paint in it and add some sand. I’d say by volume 3 parts of mud to one part of sand or a little less sand. Spread this on and then brush it with a brush the same width of what was used before. it might be a whisk broom or a wallpaper brush. You figure that out. You need the sand. You can see where the repair was made in the angle with no sand. Doesn’t match.

You may have more questions. If so ask here. Now I guess there is one more thing. If this is gypsum plaster over gypsum lath then you should use gypsum plaster to do the repair and finish it as I describe. Is this in a basement? Is there a moisture infiltration problem causing the plaster to loosen? Solve that.
 
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Old 09-25-22, 05:01 PM
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Thank you for your response! To clarify, are you saying that the deepest visible layer (which the plaster has detached from) may be concrete instead of lath? Is there any way I can figure this out? If it turns out that it is in fact gypsum lath, would it be a problem if I use the method you proposed?

The plaster layer which has detached is 1/4" deep. The inner 2/3rds of it is dark gray, the outermost 1/3 is white.

Here is my understanding of what you’re suggesting, so far – correct me if I misunderstand?
1- Remove loose stuff
2- Wire brush exposed concrete
3- Wash with 50/50 vinegar/water, dry. Question: what do you mean by "a way to handle the water"? Could I spray a vinegar mix on and then dab it off with a towel?
4- Apply Quikrete Bonding Adhesive to concrete
5- Apply Mortar Mix. What type should I use? Would this work: https://www.homedepot.com/p/Quikrete...4120/100318469 ?
6- Mix Bonding Adhesive with Joint Compound. How much bonding adhesive? Would this joint compound work: https://www.homedepot.com/p/USG-Shee...E&gclsrc=aw.ds
7- Add sand to joint this mixture. When you say sand, do you mean literally just regular sand? Sorry, I am clueless if you haven’t figured that out by now, haha!
8- Apply that mixture with an appropriate width brush.

To answer your last question, it is located at ground level, not a basement. It is on top of concrete slab. We believe the cracking occurred due to settling. We should investigate more into a potential underlying moisture problem.
 
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Old 09-25-22, 06:08 PM
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Hit the surface hard with a hammer. If it is lath you will put a hole or at least a dent in it. If it is concrete it will make a ringing sound and be unscathed.
If it gypsum lath you should use gypsum plaster. Cement does not bond well to gypsum lath.
I was estimating the thickness. Procedure the same just takes less material. If you scrub it with a vinegar or acid solution you need too rinse it. I mean scrub it with a good stiff brush. Saturate it. The rinse it with a hose. Wire brush is in lieu of washing. More work,less mess. I am still holding out for poured concrete base. I can tell you about plaster if indeed that is what you have.
There areother brands of bonding agent Quick Crete is just pretty available. Let the bonding agent dry to at least tacky before you plaster the wall. Remember If this is RockLath you need to tell me and I’ll tell you what to do.
This is a better option for your mix
https://www.homedepot.com/p/SAKRETE-...1033/100676107
Or this https://www.homedepot.com/p/LaHabra-...2502/206969849
or this https://www.acehardware.com/departme...E&gclsrc=aw.ds. The stuff you show in the bucket will work but you have to work quickly and it is harder to screed it off flat. Wet your straightening, flattening board with water. This is especially needed for the stuff in the bucket.
For finish the green lid mud is fine. By volume about t parts mud,one part bonding agent and 1 part sand. If it not sandy enough add a little more. If too sandy, well hold back a little mud before you mix it in case you get it too sandy.
I would spread on a coat f the finish then just brush it to achieve the texture. You can apply it with a brush, I suppose
Sand: yes just sand like play sand in a bag or get a gallon of mason sand or plaster sand at a masonry supply yard. they should give it to you. Screen this through a window screen. use what passes through the screen. This is the sand you want, but try to get a gallon or so not a whole bag. https://www.homedepot.com/p/1000-lb-...8069/202277789

If there is some plastering or stuccoing or brick laying going on in the neighborhood they should give you a gallon of sand.
 
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Old 09-26-22, 03:41 AM
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Ratio for finish texture. Mud: bonding agent: sand. By volume. 3 : 1 : 1.
The bonding agent is to replace the glue in the mud that is diluted by the sand. You can use latex paint in lieu of the bonding agent. I just figure you have the bonding agent and just as well use it up.
 
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Old 09-26-22, 04:15 PM
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The bottom layer is in fact concrete!
 
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Old 09-27-22, 06:24 AM
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Your pictures look like maybe more plaster could come off. Just as well do it all now rather than wait.
Let us know how it turns out.
 
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Old 10-09-22, 05:07 PM
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After applying the Quikrete, how long would you wait to apply the Joint Compound? I ended up using QUIKRETE Quick-Setting Cement (which is what the sales person at Home Depot recommended) but now I am wondering if this will work at all since it is not "mortar mix"...
Do I have to wait 28 days for the Quikrete to cure?
Thanks!
 
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Old 10-09-22, 05:18 PM
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You can finish it now. IF you think the base coat you -it on is sucking the moisture out of the JC too fas you can mist the base coat. If you need it too e a little softer in order to match the texture you can moisten the base coat.
 
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Old 10-09-22, 06:32 PM
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How would I know if the base coat is sucking moisture out of the JC? What would it look like, feel like, what would I see, etc?
 
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Old 10-10-22, 09:45 AM
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Also, do you / anyone know if it's a problem that I used quick setting cement (and not mortar)? Should I tear it out? :\
 
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Old 10-10-22, 11:31 AM
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I thought I posted a reply. it does not show up. So if the wall is sucking the moisture out for the finish it will just not be as workable. A trowel full will not seem to spread as far or you won’t be able to spread it as tightly (tight means a th in layer. No harm here, after all you want it to dry. Just you want it to dry so you can do the texture before it is too hard to do it. You will know if it is drying too fast
What you used on the base is fine. Tap it lightly with a screw driver handle or something. If it is not bonded I think you will know. Remember that ringing sound when you hit the poured concrete? It should be similar to that, If it sounds hollow then maybe it did not bond. The stuff you used has some extra bonding power — should be fine.
 
 

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