Here’s a conversation about plaster with an owner/builder that may answer some of your questions!
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.