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The right kind of plaster for finishing a cob thermal mass

 
Posts: 55
Location: Vermont, USA
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Hello all,

I'm getting somewhat close to the finish work on my thermal mass (posted a couple pictures of current state https://permies.com/p/950111) and I'm trying to figure out how to finish (as in "make durable and nice looking") it.  So far plaster seems like the main contender (helpers around with lots of plaster experience).  What I'm not sure about is what kind of plaster is right for the job.  I've seen some posts in the forums discussing gypsum (not ideal for high temperatures, but the mass shouldn't be high temperature...) and some references to lime plaster.  We've also been discussing hydraulic lime plaster as an option.  I think the gypsum plaster around here has perlite in it, too.

So, considering the mass specifically is there a reason to prefer one kind of plaster over another?  Again, this will likely be the final layer (except for any cushions or pillows or whatnot we end up with) so I want something that's going to stand up to some abuse.

Thanks,
Jean-Paul
 
Jean-Paul Calderone
Posts: 55
Location: Vermont, USA
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Well... We decided to go with lime plaster because we think that's going to be the most durable finish (and the putty is mixed up w/ the color now so probably going ahead with this unless someone points out it's going to be totally catastrophic...).  I have a related question though.  Should the lime plaster go directly on the cob or should there be a layer of something between - for example, I've heard of people doing a layer of clay slip or of lime wash.  Is one of those a good idea (if so, which)?

Current status: https://imgur.com/74k30dQ

Thanks.

 
pollinator
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Location: Penticton, Canada
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It is recommended to wet the surface of the cob first before applying the plaster (and then periodically as you work) as the cob has a tendency to pull moisture out of the plaster and possibly separate the bond leaving a weak connection.
 
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