[Cob] smoothing wall in preparation for plaster

[Shannon Dealy]

On Mon, 4 Aug 2008, Bob Smolen wrote:

> I put up my first cob wall. It is infill for a timberframe structure I 
> built.  It is 8 in thick and has dried for a little over 1 week. The 
> surface is very rough because I was rushing to beat rain when I put up 
> the wall. Is it possible to go back with a strawless mixture of cob to 
> smooth over rough spots?
> I tried doing a little and am wondering if 1/8 to 1'' "after plaster " 
[snip]

A one inch variation in the surface is not that big a deal other than 
being more work to smooth out during finishing than it would have 
been to correct while making the wall.  The high points can still be 
trimmed using a machette or (if it is not to hard) a cob saw.

I wouldn't use a strawless mixture to "smooth over the rough spots" as you 
will want a surface that has better strength/integrity to put your 
finishing coats on.  Instead, make a cob mix for coarse plaster work, 
essentially the same mix as you used for building the walls only somewhat 
finer and more consistent ingredients.  I would run the sand/clay through 
a 1/4" mesh screen, and use shorter straw.  Bales these days tend to have 
alot of short straw in them so I just run my cob straw over a 1/2" mesh 
screen before using it regular mixes, this leaves me with lots of short 
straw for plastering (a fair bit of it will be longer than 1/2" which is 
just fine for a rough plaster).  When applying, you can probably fill in 
small areas that need 1" in a single pass, but larger areas will need to 
be done in two or three coats to build it up.  For greater strength / 
stronger binding, I often add 10 to 15 percent lime putty my base plaster 
mixes as well.

FWIW.

Shannon C. Dealy      |               DeaTech Research Inc.
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