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> Guidelines for placing successive coats.
Q: Plaster is placed in two or three layers, or “coats.”
Are there guidelines or restrictions on placing each successive
coat?
 A:
Industry documents contain guidelines for placing each coat of stucco
in two-coat or three-coat systems. The recommendations are there
to ensure 1) proper curing and 2) strength development of the previously
installed coats.
To cure cement-based materials means to allow them the opportunity
to hydrate, and this requires both adequate moisture and proper
temperature. The International Building Code (IBC) provides
rules for the minimum time delay between each coat of plaster (see
table). Exterior plaster has more stringent requirements because
it is typically subject to more severe exposure than interior plaster.
Time Delay Between Each Coat of Exterior Cement Plaster Based on
IBC Requirements:
| Coat |
Moist curing (minimum period) |
Minimum interval between coats |
| First |
48 hours* |
48 hours* |
| Second |
48 hours |
7 days |
| Finish |
— |
— |
| *An exception to these time limits
occurs with a method called double-up or double-back. See below. |
There are two related, but distinct, reasons for delaying application
of a subsequent layer of plaster on top of a previously installed
layer. One reason is that there has to be adequate moisture retained
in each layer. Usually, this is accomplished by misting the plaster
once or twice during the day. It can, however, be adequately accomplished
by preventing evaporation of moisture from the plaster—and
placing the next layer of plaster on top of the other does that.
But it’s only allowed if the underlying layer has gained adequate
strength. That is the second reason for a time delay between coats:
making sure the plaster isn’t damaged by application of another
layer.
This exception to the rule—referred to as a double-up or
double-back procedure—allows for more efficient labor practices.
The wording indicates that a second coat of plaster (brown coat)
can be applied to the first base coat (scratch coat) as soon as
the first coat has attained sufficient rigidity to receive the
application without damage. This often occurs on the same day
(see the footnote to the table above). Curing of the second coat
then proceeds per the table.
Following guidelines for curing and delay between coats usually
result in the most consistent surface finish—uniform texture
and color.
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