Bonding stucco to concrete

Ask the pros-January, 2003
By Reggie Bullard


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?

                       IBonding stucco to concrete
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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