exterior wall protection

[christopher j. meyer]

(edited)
~The problem with sealing ANY wall is that the seal is never perfect.  Over
~time moisture will get into the wall (even a solid cob wall).   Cob is
relatively immune ~to these problems, but sufficient moisture will weaken a
wall.  There are however other ~reasons not to seal a cob wall, for maximum
strength you want the
~cob to dry completely (which can take 1 - 2 years), in the mean time there
~is no reason that you can't plaster the wall as long as the plaster
~breathes and allows the cob to continue drying.

~Shannon Dealy
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As I said, I may be exposing my ignorance, and I did.  I've recently spoken
with a mentor of sorts about this subject of plastering.  He has many years
of experience mostly in historical restoration, first hand information.
Some years ago there was some work done on the Rancho de Taos, not by him I
might add.  Well, portland cement was used as the cementitious binder of
the plaster.  The building was adobe.  Result:  Rancho de Taos was damn
near destroyed.  As Shannon stated, all walls will allow water to penetrate
in some fashion.  This is what happened, but because p.c. plaster does not
breathe well the clay of the adobes was greatly weakened due to the effects
of moisture.  This was taking place behind the plaster for some time, where
nobody could see the resulting damage.  Evidently lime based plasters
breathe the best, maybe a matter of opinion, I don't know.
Compatability of materials should also be considered, ie. sulfates and
concrete, which I wrote about;  and shrinkage/expansion.  Clay which has
been thoroughly dried will expand during its lifetime due to something as
slight as humidity.  Concrete will shrink, fact.  Definition: differential
movement.  At this point in time I'm led to believe that p.c. should not be
used in plaster over anything but concrete block.  Although, I understand
some people put a little p.c. in their plaster to quicken the set time.
Maybe clay plaster with wood fibers should be used over cob.  Sounds like a
pleasing combination for the ultimate in material compatability.  You may
have to replaster every so often ( I wonder about material lifetimes).

Hydrated Lime:  ASTM C207

                        Type N:  (normal)  no limits on oxide content, type
NA is air entrained

                         Type S:  (superior)  limit on oxide content, type
SA is air-entrained

                        note:  oxides are what causes lime to swell when wet

I thought that might be of some interest.