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Community and Q&A

Plaster on poured concrete interior wall?

YPwYp6LWXm | Posted in PassivHaus on

What kind of plaster can I apply directly to a poured concrete wall? Insulated on the outside, new construction. Do I need something that breathes and gives with the wall as it dries out? Should I wait for a period of time after initial construction for walls to dry out?

Thanks—- Greg

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  1. GBA Editor
    Martin Holladay | | #1

    Here is a technical manual that should answer your questions:
    Gypsum Plaster Over Concrete.

  2. Natur Haus | | #2

    You can apply any kind of plaster over concrete, the key is to reduce the hydrophilic suction of the concrete via plaster bond or a simple sealer. Plaster systems that are constructed of more than one coat will also work better. What type of look or material are you going for?

  3. YPwYp6LWXm | | #3

    I am going for an " old world " probably troweled look. I am confused as to just what kind of plaster I should use. Just found RED TOP " gauging plaster " at a local supply shop. Verry little knowledge as to this kind of aplication around here. Seams like a simple thing to do but --- what kind of plaster, prep , and application, how to color. Thanks for the input.---- Greg

  4. Natur Haus | | #4

    I wouldn't call gypsum plaster "easy" for a beginner. Gypsum plasters chemically set and do not "air dry." There are a few options when it comes to plastering your wall, gypsum plaster, lime plaster, earth and clay plasters, and of course sticking with cement based plasters. The lime and gypsum plasters are strong, durable, and come in mixes that range from single coat "Gauging Plasters" to two coat "veneer or Venetian" systems. These two coat systems are very smooth, and the single coat systems would have more surface variation, in general. The clay plasters will give you an earthen look. I have used a lot of different plasters, and for a beginner, I would recommend American Clay plasters. These plasters "air dry", so they don't have immediate time constraints. These plasters are not as durable to impact and abrasion as the gypsum and lime plasters. A hydrated lime plaster could be another user friendly plaster.
    Two questions: Do you want the look of earthy or refined?
    How smooth is the poured wall?

    BTW, Red Top Gauging plaster could be used as basecoat(with aggregates) and as a finish(mixed with lime), so depending on what is available to you, could be versatile in it's use.

  5. Natur Haus | | #5

    Are you still there Greg?

  6. YPwYp6LWXm | | #6

    Hi Matthew. I am still here. After looking up the different types of plaster I still have some questions. Is the Red Top gauging plaster breathable? It seems that this is a very good quality to have in a plaster. The Red Top is cheap.. $ 15 a 100 lb. bag at a local shop. Can it be colored and with what? The Lime plaster sounds very good and I beleave that it is breathable. Did not find this locally and don't know the cost. What is the difference between hydrated and hydralic lime plaster? Found an article where they were going back and forth about the merits of both with no conclution. The American Clay plaster looks like a neat product. They have a new procuct that you roll on and then work to your desiered finish. $ 70 for a 50 lb. bag. This stuff is brathable also I think. The walls have not been poured yet. What would be the best base when the forms come off for the plaster to adhear to and what would be the prep for the walls?Should I let them cure for a time and how long? I am looking for a earthy/old world/ look for the finished wall. Thanks---Greg

  7. Natur Haus | | #7

    Thanks Greg, now we got something..... Red Top is breathable...... It would be tinted with earth pigments, or you could use the powdered concrete pigments that I would imagine are available in your area. If you are going to use gypsum or lime plasters, you should seal the concrete with plaster bond or any simple acrylic sealer, smooth wall-plaster bond, rough wall-sealer could work. I know guys that water down elmer's glue as a sealer/bonder. If you are going to use American Clay or another air-drying plaster, you could use some simple paint primer, with or without sand, or a coat of simple acrylic sealer to cut absorption some, or conceivably, just wet the concrete thorough for a several hours before to cut the suction of water out of the clay plaster basecoat. I would recommend one of the first options. If you want it to be durable and tough, go with the gypsum-lime, if you want he truly earthen look, go with the clay plaster.
    The concrete wall should be fully cured and dry before you seal and plaster it. Cost wise, I make my own clay plaster, which makes it cheap. The Red Top-Lime plaster would be pretty cheap, and pigments aren't that expensive.
    My plastering contractor friend( said that he would mix Red Top with Lime and silica sand for the first coat, maybe a 50/50 of gypsum/lime. For the second coat probably 75/25 Lime/Gypsum. These ratios could be played with, keeping in mind that the Gypsum has a quicker set and hardness and the lime a slow set and slower hardening. I'm sure we'll be discussing this further, but this is hopefully a good piece to chew on. By the Way, the lime I am referring to is Type S lime, which I'm sure is available where you are.

  8. YPwYp6LWXm | | #8

    Just to be clear .The Red Top Gauging plaster could be used as is, after a bonding agent was applied to wall ? Would you mix the pigment dry or after the water was added? I have attached a couple of photos to give you the texture I am going for. -------- Greg P.S. How long does it take to cure the concrete wall. 12" thick. Rough estimate. Weeks , Months?------- Greg

  9. YPwYp6LWXm | | #9


  10. Natur Haus | | #10

    The photo helps Greg, Thanks. Yes, the Red Top could be used as is, with a mason's sand or equivalent in the basecoat, and fine silica sand in the finish coat. Mixing it with Lime in either coat will slow the set of the gypsum, which would help the workability for you I believe.
    I would mix the dry pigment in the water first, then add dry plater mix to the water. When you are spritzing(with water) and troweling the finish coat, it will give you a slightly lighter tone. Where it is spritzed and troweled less, and where the under coat shows through in the recesses, it will be slightly darker. This will give you the look your going for.
    As far as the concrete cure, I would say 30-60 days depending on relative humidity and temperature.

  11. YPwYp6LWXm | | #11

    What proportions of sand would you use and to what effect would more or less sand change the finished product. Also rough proportions of lime and the effect that has. What about time between coats? Anything specific between coats or could I do a whole wall with first coat and then come back and do second coat at any time? This mixture seems to be what I am looking for. Is this the basic mix that has been used for centuries? I think that is what I need. ---- Thanks --- you have been a great help. ---- Terminoligy was throwing me.---------- Greg

  12. Natur Haus | | #12

    Greg, sorry for any confusion, but it happens easily with the plaster subject. This art has been all but lost. BTW, for some great info, picture and possible answers, visit my buddy's website:
    As far as the sand question, in the first coat I would think between 1:1 lime to sand and 1:2 lime to sand.
    The finish coat would be the same but the grit would be more in the 60-90 grit range. The 60 grit would give you the old world look, and with plenty of cream to pull to the surface when finish troweling. You can plaster it again at any time, but USG would tell you that if it has been more than like 24 hrs that you would need to use plaster bond again. You should verify and double check all of this info with the USG Red Top tech specs, keeping in mind that there are a lot of rules you can break and be fine! I have broken many of them! I would also recommend you using a pool trowel if you are a beginner.

  13. Natur Haus | | #13

    BTW Greg, that picture you posted of Tadelakt is a lime plaster, my buddy talks about and support this on his website. Again you could use the Red Top only, a combination of Red Top and Lime, and Lime only, there is no magic formula. The old schoolers used to mix and match a lot to address given situations, as they understood the chemical/mechanical properties of these minerals. Check out these websites and feel free to ask further questions, you could study for years and still have them :>)

  14. [email protected] | | #14


    I have a similar project in mind, I would like to plaster then texture over old uneven concrete. I saw a product called plaster-weld. Can I use the plaster weld over the poured concrete wall and then just layer on coats of joint compound to get a textured even finish, then seal with a primer and finally paint it? When they formed up the basement walls years and years ago, they weren't concerned with even finish. I can send you a photo if you need one. We get very little moisture in basement

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