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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.
Click
here
for more on Milestone.
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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