A Conversation About Plaster with an Owner/Builder

Written by Andrew Morrison

pastering straw bale houseHere’s a conversation about plaster with an owner/builder that may answer some of your questions!

Dear Andrew,

We had quite a few days applying the “Scratch Coat” to our straw bale structure. It is so true that the whole process is really about having one stage done as completely before the next stage. But you are the best advocate for this.

Having applied the “Scratch Coat” there are a few questions:

Q: It has been a week since the “Scratch Coat” was applied, how much longer do I mist down the walls and how often?

A: You can stop misting after a week but the longer you mist, often enough to keep the plaster moist, the stronger the cure will be.

Q: What is the optimal time to wait before the “Brown Coat” is applied?

A: Again it is about curing. The longer you wait the better. After you stop misting the walls, you want to wait until the plaster dries out and watch for cracking of the scratch coat. Once the plaster is completely dry, you can prepare for the brown coat.

Q: Is there a best practice for removing plaster from wood surfaces that were accidentally touched?

A: This is tough because the plaster will likely have stained the wood. The best option is to use vinegar to neutralize the lime. Be sure to check the reaction on the wood some where hidden before you go to town with this option!

Q: Is there a long range issue about the effect of lime plaster as applied to raw wood surfaces? Is there an antidote?

A: See above. That trick is the best option for stain grade wood and paint grade wood. If you go with paint grade, you may need to add an additional primer coat. With stain, additional sanding may be required to get under the stain.

Q: I have left over 10 gallons of well mixed plaster in a tub covered with plastic. Does this mixture need to be put through the mortar mixer before being applied or can I apply it direct?

A: You will need to re mix it when you are ready to apply it. Keep in mind the brown coat is a different ratio of sand to lime so you may want to mix a little bit at a time into new batches of brown.

Q: Can I incorporate the left over mixture with a new batch?

A: Yes, as long as it is mixed well and consistently. It is important the mixtures be as close to the same as possible for each batch of plaster so if you are adding old mud, add it equally to the new batches, not all at once. You cannot do this for the finish coat. It will need to be its own mix and the color quantities have to be exact in each mix.

Q: Is it ok to mix a small batch and complete sections of the project as my time and schedule allows or is it advisable to do the whole outside and whole inside at the same time?

A: You can do it over time. Be sure to locate a smart stopping point. I suggest you do entire walls and stop at the corners, actually 6″ away from the corners. This way you can feather the continuation coat into the existing coat. If you stop in the middle of the wall you will have a burn mark and a cold joint.

Q: Renting a mortar mixer has a cost factor I would like to avoid. I am planning on do the brown coat and finish coat by my self on the weekends. Rather than rent mortar mixer every time I am plastering can I mix all that I will need slack, store it, and use it as I go?

A: Sort of. You need to re mix the mud before you put it on the wall once you have slaked it. You can remix by hand, but I think you find it extremely difficult to do as the slaked plaster tends to get fairly hard.

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6 Responses

  1. We have some concerns regarding the Lime render brown coat as well. We have just ordered your DVD (wish we’d known about it 2 years ago!!) We have an area where a hard freeze snuck up on us after just applying the brown coat. There are places where ‘spalling’ has occured- the render turned powdery on the surface. Is there a ‘better’ way to fix this than tearing that whole section off and starting over? Thanks.

  2. Chris,
    I am by no means a plaster expert, but I do know a thing or two about it and the basics of how it works. It would seem to me that as long as the attachment of the render to the bales is in tack and you are just experiencing surface spalling, you should be okay to simply redress the surface by knocking off the loose particles and re rendering the surface. If the attachment was affected as well, then you will need to remove all of the material that has failed and re render the bales. I would assume that the only impacted area is that which was exposed to the cold. The fact that plaster creates its own heat through a chemical reaction as it cures should have helped the deeper layers of the render survive the cold spell. So I expect you can simply scrape the loose material from the surface of the wall and start over. The scraping will likely cause some depth in the plaster which will act as adequate tooth for the new layer of render to grab onto to. I don’t suspect this will be a major pain to fix, although you probably thought you were done the first time! I hope this helps. Please let me know how it goes.

  3. Hi Andrew, I have heard that applying a linseed oil / turps mix to the finished lime render will prevent any loose powdery bits from rubbing off. Any comments?

  4. Hi Carol, I have heard of this as well but never done it. It also creates a nice leathery finish when burnished into the plaster with a steel trowel or smooth stone.

  5. I have completed my scratch coat and let it dry slowly for 10 + days. I recently patched some low spots with plaster to even out the walls in a few places and I’m wondering how long I have to wait to apply the brown coat in these areas. Thanks!

  6. Technically, you need to wait the full 10 days. You can do what is called a secondary scratch over the initial scratch coat to fill low areas withint the first day or two of applying the initial scratch and then let the whole thing sit for ten days. In other words, don’t wait the first ten days to apply the filler material, just wait one or two and then let the WHOLE THING sit for 10 days.

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