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FAQ's > Bonding agents
Q: Is it necessary to use a bonding agent with
stucco?
A: Products that increase the adhesion of plaster
to substrate or plaster to plaster are called bonding agents, and
are either surface applied to a substrate or integrally mixed into
the plaster.
A distinction should be made between framed construction and solid
backing (such as masonry or concrete). Framed construction requires
the installation of moisture-resistant paper behind the lath. You
do not want or need to have plaster bond to the paper, so bonding
agents are not used with framed construction, only solid surface
substrates.
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| When plastering over concrete masonry, walls
should be free from contamination that might impair bond. |
If contamination is present on the substrate surface, good bond is inhibited.
This is generally not a concern with new masonry walls, but can be
an issue with new cast-in-place concrete as it may have residual form
release agent on its surface. Older concrete or masonry walls may
have bond-inhibiting characteristics, in the form of paint, sealer,
some other coating, or dirt on the surface. As such, bonding agents
are more likely to be considered for repair and renovation work over
either concrete or concrete masonry.
It is generally good practice to prepare the solid substrate so
a bonding agent is not necessary. The prepared surface should be
clean (all surface materials removed), sound (hard surface), and
mechanically roughened. Methods for achieving these criteria include
sand blasting and high-pressure water blasting. When this type of
preparation does not result in a clean, sound, and roughened substrate,
bonding agents offer another solution.
Bonding agents have different chemical formulations, so they have
different performance characteristics. Bonding agents do not guarantee
performance. You will need to research the material to find out
which is best suited to your particular conditions. But where prepared
surfaces seem questionable, and lathing is not an option, a bonding
agent may be beneficial.
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| Plaster finish on a concrete masonry wall. |
Surface-applied bonding agents should conform to the requirements
of ASTM C 932 (www.astm.org).
Integral bonding agents should be used only after review of the
manufacturer’s documentation of testing and past performance.
Dash-Bond Coat
Rather than using bonding agents, another option for low-absorption
surfaces is to apply a dash-bond coat. This cement-rich slurry is
dashed against the base surface by hand with a brush, trowel or
paddle, or by machine. Most of the surface is covered with the plaster.
The high cement content provides a tenacious bond. This material
is left unfinished so that a rough base is created for the scratch
coat.
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