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New plaster cracking

Brian Abbott | Posted in Construction Techniques on March 19, 2009 05:14am

I completed (or thought I completed) my very first plaster job the other day. I began by applying basecoat plaster over monolithic concrete, and then several days later put on a coat of veneer plaster over that. However, 3 or 4 days after applying the veneer coat, long, meandering hairline cracks starting appearing. By looking through earlier threads I found some useful ideas for repair, –but I’d like to know what caused this . . . and if the cracks are going to get larger or continue to appear.

Thanks in advance, –Brian

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  1. barmil | Mar 19, 2009 05:46am | #1

    If you had done this in 1936 when my home's walls were plastered, you wouldn't have this problem. Flippant answer, I admit, but there must have been a technique at that time that maybe isn't as known or apparent now. My plaster walls aren't over concrete, which is quite able to transmit moisture. Do you think this could be the problem?

  2. oomingmuk | Mar 19, 2009 05:47am | #2

    I build strawbale homes and I'm not a plaster pro by any means - but we do plaster the houses we build. I had a similar problem with a cement:Lime plaster job recently... the hairline cracks could be from overworking the plaster, a poor mix, and in our case, the plaster dried out too fast. The substrate (monolithic concrete for you, strawbales for us) needs to be fairly saturated or it will suck/wick the water from the plaster and cause it to dry out too quickly - thus forming the cracks. Our cracks grew over the first few weeks, but it's a new building and we expect cracking as the house settles during the first year. Standard practice for straw building is to go back after a year to do the final/finish coat, repairing all the cracks at that time. However, I was chatting with a stucco guy from a build next door and he puts hostapur in his mix - it's an air entraining admixture - he said ever since he started doing that, he's never had a problem with cracking...

    1. Brian Abbott | Mar 19, 2009 06:38am | #3

      You both (oomingmuk and BARMIL) seem to be suggesting that perhaps the problem was the plaster drying out too fast. Maybe, but I sure tried hard to avoid this. I put a bonding agent over the concrete before applying the scratch coat. Then I sprayed down the scratch coat with a garden sprayer before applying the veneer coat. After the veneer coat was applied I could see it was drying way too quickly (even though I was in a basement with temps in the low 60's), so I sprayed it with the garden sprayer at least a dozen times or the next hour or two to try to slow this process down. When I came back the next day I was delighted that there were no cracks. It wasn't till the next week that they started to appear...

  3. Jer | Mar 19, 2009 01:38pm | #4

    Drying too quickly is usually the #1 reason. You're also jumping from portland to gypsum. How old is the portland? If it's still green then you might be experiencing the portland's activity through the plaster. If the portland is cured, then you might try painting it with a sealer first, then the boding agent (plasterweld) before applying your base of plaster.

    Was the base coat of plaster typical "brown coat" material like pearlite or sanded plaster? "Structolite" by USG is a typical example of this. When floated to a flat but semi-rough finish, this leaves an excellent base for the veneer. Always wet it down (not too much) before the veneer coat.

    The technique of applying the veneer is also important. The spattering & mopping process during the final floats is crucial. You have to keep it wet.

    Lastly the veneer may be bad. I've run into this many many times over the years. If you get the plaster at a mason supply where the product is moved at a higher rate and is stored in a dry area then you're usually ok. If however you get it at a place like a lumberyard or hardware supply where plaster is not a big item and they may have some bags that have been sitting around for a long time, you're taking your chances.

    Plaster is always trying to rehydrate itself.

    Plaster does not have a long shelf life at all. If you do store it, wrap it tightly in a thick plastic bag and duct tape it shut keeping out all moisture.

    1. Brian Abbott | Mar 19, 2009 02:57pm | #5

      You might be onto something about the age of the plaster. It was a partial bag I got from a contractor-friend, who bought and used a small amount of it almost a year ago. Or is there any chance I applied it too thin? It seemed to apply at its own "natural" thickness, without my making a conscious effort about it, --so I assumed this was about right. But the USG book I downloaded suggested this could be the case.This job was a remodel, and the concrete over which I applied the plaster was many years old. I did use Structolite for the basecoat.Since it's getting painted, I think I can do a satisfactory repair by working some durabond into the cracks. My main question is making sure that before the paint goes on, all the cracks that are going to form have done so, and that they aren't going to get any larger.Thanks much, --Brian

      1. Jer | Mar 20, 2009 04:33am | #6

        "It was a partial bag I got from a contractor-friend, who bought and used a small amount of it almost a year ago"Oh Lord! There's the problem! Big no-no. Always buy fresh with plaster. Never use an opened bag unless it hasn't been that long and it was stored properly. If plaster rehydrates itself at all, it loses a great deal of it's hardening properties. The degree varies depending on the age & the moisture. Plaster can seem dr, but it contains moisture to one degree or another. If the plaster that's on there now is hard and it seems to be stuck to the base coat, then you're probably ok and you did the right thing by filling in with the Durabond. Let it cure for a few more days and then try and chip it off the brown coat. If it's difficult, then you're ok, if it comes off easily, start over with the veneer, only this time, buy a fresh bag.My favorite white coat is the traditional lime putty and gauging plaster. You can spatter and trowel it until it becomes so fine that you can see your own reflection. Venetian plaster is like that, encaustica and Marmorino is the same.

        1. Brian Abbott | Mar 20, 2009 02:55pm | #7

          Thanks again. Yes, it seems hard and well-adhered, so I guess I'm OK this time. Next time I'll know to start with a fresh bag! --Brian

    2. BryanSayer | Mar 20, 2009 05:17pm | #8

      If this discussion is about real plaster (ie something lime based, rather than gypsum based) then it does not dry. Like concrete, it cures. And it is exothermic, meaning it gives off heat when it does.Which is not to say that curing too fast isn't the problem, but the solution may be different. Such as adding a retardant (cream of tartar is one) to slow the curing and increase work time.

      1. billybatts | Mar 21, 2009 03:45am | #9

        i believe that it is the plaster of paris or gauging plaster that causes lime plaster to give off heat, i know this because you can store wet  lime plaster for months or years at a time in a bucket and it will not set, you cant store POP the same way

      2. Jer | Mar 21, 2009 05:15am | #10

        "If this discussion is about real plaster (ie something lime based, rather than gypsum based)"Plaster is gypsum plain and simple. Lime is lime. Lime is not plaster. Lime is an additive, like an aggregate if you will. In white coating, slaked lime is added to plaster (gauging or POP) at a 3:1 ratio to make up a "gauging mix" that has a plastic like quality so it can be worked flat or run in a molding. Plain gypsum plaster when mixed with water does not have the workability as when lime putty is added.

        Plaster is ground gypsum rock that is baked or "calcined" to a certain degree depending on what type of plaster is being produced. The baking drives off the water that's in the rock. When the powder is reintroduced to water, the molecular structure changes into a new & different crystaline structure, the hard plaster that we know.
        The term "curing" is different than drying.
        My solution was not about curing as it was the moisture content being either sucked out too fast or the old plaster was already partially rehydrated (cured) and therefor produced an inferior, weaker material.Indeed you can put an additive into the water while mixing to retard the setting, USG makes one that's a powder, or cream of tartar is used, I have also used white vinegar,...any kind of common alkali, but prefer to use only a little or none at all if I can use it up fast enough because the retarder weakens the plaster a might.The word "plaster" is used generically often and a lime & silica sand is also called "whitewash", it, along with other binders was also the mortar for bricks before the age of portland cements.

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