[Cob] Re: was black asphalt, now plaster over ply/paint

[Barbara Roemer and Glenn Miller]

Brent asked:
> 
> Does anyone have experience plastering conventional sheetrock walls that
> have a paint coating, is it possible?  What abou plastering plywood?  snip

First and last advice - make big samples on the walls you plan to plaster.

While you can plaster over just about anything, the bond between earth and
earth or straw and earth in all its forms is strong.  There is almost no
bond between earth plaster and a painted surface or a sheetrocked one, so
providing tooth for the plaster to adhere to is essential.  I have plastered
three surfaces in my home, and they've been in place for three years now.

Over new sheetrock, the best way is to apply the sheetrock backwards, that
is with the dark side out and the light side facing the studs.  It's harder
to get seams to be invisible this way, but entirely doable if your plaster
mixes are thick enough and sandy enough.  the grey side has more tooth.  My
sheetrock was already up before I discovered this, and it's fine, too.  The
next thing to do is to roll on a coat of wheat paste.  You can skip this
step if your plaster contains plenty of wheat paste (and adding casein helps
adhesion, too).  The down side to wheat paste is that a lot of it (say one
part of your mix with 3 parts clay, 4-6 parts sand, all depending on how
sticky your clay is and your test patches) makes the plaster more difficult
to work with.  It changes color slightly as you trowel it so if you have a
troublesome area where you have to work the plaster more, it tends to looks
kind of greyed there as opposed to areas you didn't work as much.  I'm
pretty fussy, so this might not even be obvious to others.  It shows more on
a smooth plaster job than a rough one, too, and if you are doing an alis or
clay paint over the plaster, then it won't matter at all.  I taped my seams
with the polytape because the paper tape pulled up too easily in the
moisture and long-drying of the plaster.

Over plywood, you must provide something to hold the plaster.  Burlap (maybe
from old coffee bean bags) is the best.  Hammertack it to the ply and start
plastering.  This technique works great where cob and wood members meet,
too. Again, use plenty of wheat paste in your plaster, and really push the
plaster into the burlap.

For sheetrock that's already been textured and painted, it depends on the
smoothness or tackiness of the surface.  I am currently redoing a knock-down
sheetrock texture on ceilings which are painted with high gloss paint.  A
combination of factors allowed the earth plaster to work directly on the
painted surface and adhere well.  I have extremely sticky clay (very high
clay content), the mix is stiff with sand, has some straw in it, the ceiling
has very thick texture so there is plenty of depth and places for the earth
plaster to catch, and I added lots of wheat paste because the surface will
be clay painted.

More questions, email me off-list.  Final note - if your test patches of
plaster are cracking like eggshell pottery, use more sand or it will flake
off of whatever you're applying it to.