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Lime plaster questions

 
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Hello all,

We are from the Midwestern USA, but living in South America mountain climate, and building a timber frame construction with wattle and daub.

We are ready to do lime plaster on the exterior daubed faces.  Rains have set in.  The tin roof has a 1 meter overhang, and we are happy to find rain splatter is minimal.   With cloudy, rainy weather setting in, I think the cure time of the plaster will be slowed down enough that we don't need to do anything special to slow the process.

Instructions for applying lime plaster seem to vary a lot, but most seem to suggest 3 coats.  On the other extreme, it seems that a well-protected daub wall could be finished with just a few coats of whitewash.

So we are wondering,
1. Can we count the daub as the scratch coat?
2. Can we do just one layer of plaster followed with a thick whitewash?  
3. Opinions on adding salt?  What I gather from research is that, being hygroscopic, it helps the plaster bond better to clay, but shouldn't be used over masonry.  Can help slow drying but isn't necessary on that count (I gather), especially with our rainy weather.

We're starting the first layer of plaster tomorrow, we will do it about 1 cm thick worked into the clay.  From practice patches, I know streaks of clay may show up.
Thanks for any suggestions!
Hoping we can get some ideas about whether further layers are important before whitewashing.

 
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Location: Ashhurst New Zealand
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Hi Hannah, and welcome to permies!

When I've done lime plaster over cob, I like to do a thin limewash as the first coat. This stabilises the earth and penetrates to give a better chemical bond for the plaster. I've never heard of using salt and in a humid climate that would give me pause. I wouldn't want salt crystals down inside the plaster coat to be absorbing moisture and creating little zones of saturation that might lead to delamination. But I haven't tried it and maybe there's something I am missing in the technique.
 
Hannah Johnson
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Hi Phil,
Great, we'll run with the whitewash as a first layer. I have been reading as I can, but this is my first time going into action with whitewash.  

Would you mind confirming approximately what proportion of water to lime putty you use in a whitewash? By default from other reading I'll add water until it is like milk, to get the crew started this morning, but I'll watch my phone.

Am I right to assume we'll still want to moisten and score the clay before brushing on the whitewash?
 
Phil Stevens
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Skim milk. You want it on the thin and watery side so that it soaks into the earth before it coagulates and skins over. I don't have an exact ratio, as I do my mixes by eyeball and feel. Do some test patches to get a sense of how the materials interact.
 
Hannah Johnson
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Thanks for the tips on this a while ago.

I am sure we did not do it perfectly but we are living in our house now!  (and like many DIYers I've known... it has a ways to go, haha) It may be that down the road we have to redo the lime plaster.  While the workers were doing that part of the project, I was near to giving birth/shortly post-partum. I wanted to be more involved, but had to just boss the guys around a lot through my husband, who was involved on-site but not doing the research!  

Mainly the reason I think we could have to redo the plaster within a couple of years-- while the guys did wet the daub before applying the plaster, they did not work the first layer into the clay like I had asked them to.  Then, exterior door frames and floorboards were installed after the lime plaster was applied.  The result is that there was a lot of violent pounding on the house. Next to one door frame, about a square meter of plaster fell away.  Granted, that was the "experiment" section of wall where everything was always done worst, being the first time.  The guys, who had tried convincing us to use quick concrete for an exterior plaster, assured us the same thing would have happened with a concrete coat, as hard as the carpenter was pounding boards into place.  

Besides the little part that fell away, however, there are hairline cracks all over the place.  This isn't immediately causing problems, but we'll at least be thoroughly whitewashing the house again before next rainy season.  It's so beautiful how whitewash heals those tiny cracks!  I just don't know what that means in the long run for the plaster in general.  I'm preparing myself psychologically to redo the plaster in a few years, but hoping it holds up in spite of apparently not being keyed as well as it might have been.  Living in a equatorial mountain climate, we don't have deep freezes to worry about.
 
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