Sacramento Valley Bureau for Lath and Plaster
1400 S Street, Suite 203 • Sacramento. CA 95814
Phone: (916) 444-2397 • Fax: (916) 444-6635
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Contents:
Ÿ
ASTM C-926 Environmental
Conditions
Ÿ Curing
Ÿ Interval
ASTM C-926 Environmental Conditions
Portland cement-based plaster brown coats should not be applied to
frozen scratch coats, and finish coats should not be applied to frozen
brown coats. When the outside temperature is below 35 degrees it is not
wise to plaster without tenting the building or adding up to 10% Calcium
Chloride. 40 degrees and rising outside temperatures normally do not
affect the set of the plaster. The most conservative wintertime
parameters should include starting plastering as the temperature rises
from 35 degrees and stopping when the temperature starts to drop. This
will assure that the applied plaster will set before it freezes.
Freezing kills the set and the plaster must be removed and replaced.
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Curing
The curing of plaster is a process some have determined to be 28 years.
But it gains most of its strength during the first 7 days. This depends
on the type of cement. Common Cement seems to cure faster than Plastic
Cements. No plaster will cure without moisture, either added back into
the plaster, or retained in the panels. The longer moisture stays in the
plaster the quicker it cures. Moisture leaves plaster by evaporation.
Hot days and windy conditions can suck the moisture out of a plaster
panel in less than an hour.
Curing stops when the panel is dry. It starts again when the panel has
moisture added to it. This process of moist curing and for how long is
covered in Table 25-f of the 1997 UBC. We suggest moist curing at least
morning and evening, for two days for the scratch and the brown coat. If
a finish coat is applied to a dry brown coat on hot days it can flash
dry. Powdery and flaky finish is indicative of this condition.
The Moh’s hardness test is an on the job test to see if a plaster panel
meets plastering industry curing Standards. It is a scratch test that
assigns levels of hardness from diamond at 10 to Talc at 1. Cured
plaster panels generally are 4-5. A penny is 3-4. If the penny scratches
the wall it is too soft and is not cured. If the wall scratches the
penny it is cured .It is a simple test and let’s you know whether your
plaster panel is cured.
Moist curing a wall is done by opening up the pores of the plaster with
a fine mist, around the whole building, then a more direct spray from
the top down until you see water weeping from the weep holes, fully
saturating the plaster.
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Interval
The interval of time between the scratch and brown in the UBC has been
48 hours for years. However it was thought in the industry for a long
time that curing the scratch for 7 days and curing the brown for 21 days
produced a better plaster job. Many now believe that placing the brown
coat over a green scratch coat produces greater crack resistance because
the panels cure together. This is moving towards the installation
procedure contained in 2508.6 of the 1997 UBC.
In fact we have companies in Northern California that precisely mix
their plaster so that it can be applied in one pass, a full ¾”inch. The
reason for having a scratch and brown is simple, to keep the plaster on
the wall and off your shoes. It has to be spread in two thinner panels
instead of one complete scratch/brown combination. We find a better
chemical bond when the two coats are applied and cured in close
conjunction. We also find greater crack control.
