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Bonding
stucco to concrete
Hello Reggie,
I am building a castle made
from
concrete.
Here is a picture of the inside of the kitchen. If
I want to finish the
walls with a lime
plaster, do I need to do
anything
special
to get the best bond between the cement base and
the lime finish?
I
Cool kitchen.
The best way to bond
stucco mortar to
concrete
is an acrylic admixture,
such as Flex-con from the Euclid
Chemical
Co.,
or you may order
Milestone "E" chemical from
Seattle. The E
chemical
is concentrated and
can be diluted with water, so
may be more
economical.
We mix Flex-con
50-50 with water,
and then
mix
with cement
mortar. Swimming pool people
tell me 25% is
fine,
but why be cheap?
The acrylics add
far more
properties to the
mortar
besides superior bond.
Among these properties are
better water
retention
in the mix, flexibility,
and best of all, incredible
strength.
This mix can be
applied with a
trowel and
roughed
up with a float or
even a brush. You may
even try
painting
on soupy mortar with a
brush. The
important thing is the
surface
MUST BE rough.
The flex-con
bucket has other
instructions
for
a bonding coat on
concrete. You can mix Flex-con
with pure
portland
cement ( no sand),
and apply with a stiff broom,
leaving the
surface
rough.
A portland cement stucco finish
can now be
applied.
What
did
they do in the old days?
Mortar
was bonded using
a portland cement
paste.
Portland was mixed
with water (no sand or lime),
and painted
on
the concrete. To do
this first wet the
concrete, and
brush
on, leaving rough brush strokes.
VERY IMPORTANT: You will notice
that the
paste
sets instantly when
it goes on the concrete. Then
you can watch
it
dry, as the concrete sucks the water
out. You MUST apply the mortar before
the
paste dries. This requires either two
people, one brushing on the
paste
and another applying mortar, or one
person applying paste and mortar
Portland cement paste is still
used in
Latin
America, and works fine.
When I
was in Mexico, I
saw ceilings on
buildings
done this way. If
a piece of stucco mortar comes
loose and
hits
someone it could be
fatal. The plasterers who did
this work
applied
the stucco with the
confidence they wouldn't have
blood on
their
hands if their ceiling failed.
Plaster can be
bonded with a paint
on chemical bonding agent such
as plaster weld or euco-weld.
Euco weld is
the
best. Paint on without diluting.
We
always trowel on a
base coat such as
veneer
base coat before the finish coat.
The
advantage of
veneer base coat instead of
plaster
and sand is that it sets fast avoiding
dry-outs, that is where the
mortar
dries before it sets.
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