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> Correct Proportions for Different Coats
Q: What are the correct proportions for stucco? Is there a difference
between the coats?
A:The brown coat is applied over the
scratch coat to prepare the plaster base for the finish coat application.To
start with, some systems are made up of two coats and some are three.
Three-coat work is for systems that are constructed with lath and
two-coat work is for direct application to masonry or concrete backup.
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| The brown coat is applied over the scratch
coat to prepare the plaster base for the finish coat application. |
The three coats consist of two base coats and one finish coat.
The first base coat is called a scratch coat, the second is called
a brown coat. In two-coat work, there is a single base coat and
a finish coat.
The purpose of the first base coat, the scratch coat, is to embed
the metal lath and provide a base for the brown coat. The scratch
coat gets its name from the fact that it is physically scratched
with horizontal marks. These scratches create a “key”
for the next coat to grab onto and a shelf for moisture to aid in
curing of the brown coat. The brown coat covers the first base coat
and creates a plane surface, leading to the best possible results
for the finish coat. The finish coat is the thinnest of the coats,
and its purpose is to impart a decorative surface to the plaster.
Scratch, brown, and finish coats all have slightly different proportions.
Each one has a range that is allowed, but all are specified by volume.
One reason for this is to give the contractor some leeway in choosing
a mix that works best with his specific materials. Another reason
is that certain properties of the hardened plaster can be accentuated
in each coat. The first coat will provide a hard base for the system
without a great deal of shrinkage. The greater sand content in the
second base coat might generate less shrinkage to create a better
base for the finish than the first base coat. The finish coat should
be hard to resist abrasion and other surface damage.
Proportions are clearly spelled out in ASTM C 926, Standard Specification
for Application of Portland Cement-Based Plaster. [www.astm.org]
Scratch coats are mixed at 1 part cement to 2-1/4 to 4 parts sand,
brown coats are mixed at 1 part cement to 3 to 5 parts sand, and
finish coats are 1 part cement to 1-1/2 to 3 parts sand.
It is important to note that the term “cement” includes
all cementitious materials, such as cement plus lime. So if 1 part
cement is used with one-half part lime, that equals 1-1/2 parts
cementitious materials, and that total is then multiplied by the
sand number. For the finish coat, for instance, the range is 1-1/2
to 3 parts sand: 1-1/2 times 1-1/2 is 2-1/4 and 1-1/2 times 3 is
4-1/2. So if we have 1-1/2 total parts of cementitious materials,
the sand parts would range from 2-1/4 to 4-1/2.
Click here for more on plaster thickness.
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