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FAQ's > Curing stucco
Q: Does stucco require curing, and if it does, how is this best
accomplished?
A: For cement-based materials, curing
is defined as maintaining an appropriate temperature and moisture
content for a specific period of time during the early life of the
material. All portland cement-based materials, such as stucco, require
curing.
Since the addition of water to portland cement sets off a chemical
reaction called hydration, it’s important to provide excess
water to the cement particles so that they develop a good bond with
their surrounding environment: aggregate and other cement particles.
This is how plaster (and concrete, mortar, and grout) hardens. Plaster
sections are quite thin, ranging from about 3/8 in. to 7/8 in. total,
and individual coats may be only 1/8 in. thick. Thin layers such
as this must be protected from conditions that interfere with cement
hydration: things that dry them out or heat or cool them excessively.
Wet and Dry
 |
| The scratch coat, which is the first base
coat, is named for horizontal scratches that are there to promote
water retention for curing and a mechanical keying with the
brown, or second, base coat. |
Sun and wind, alone or in combination, drive moisture out of fresh
plaster. To be applied to a wall, plaster must be fluid enough to
be troweled, screeded, and floated, but not too wet that it sags
or won’t stick. Base coats, of which there may be one or two
(sometimes scratch and brown are combined), can be wetted once they
have developed adequate strength so that they are not washed away
by the water. Since the coats are thin, they can’t hold as
much moisture as is ideal for curing—especially if they are
competing with sun or wind, which both cause evaporation.
Plaster can be wetted periodically throughout the day to supply
additional curing moisture; usually one or two times per day should
suffice. In extreme conditions, sun and wind breaks can be used
to provide extra protection from the elements. The first two days
are the most critical period. The entire first week is important,
however, so it is a common recommendation that the base coat stucco
be misted or fogged periodically for the first three to seven days
after placement. A sheet of polyethylene can be placed over the
moistened surface to hold the water in. If the relative humidity
of the air is greater than 70%, moist curing may be accomplished
without additional wetting of the surface.
 |
| Colored finishes are usually cured by wetting
the brown coat to provide curing moisture from behind. In more
extreme conditions, they may be covered to prevent drying. |
A caution about moist curing is that colored finishes can be affected
by water application. Finish coat stucco is not moist cured since
this may promote mottling and discoloration. Curing of colored finishes
is typically done by wetting the base coat to provide curing moisture
from behind the finish and ensuring that the surface is shielded
from drying.
Hot and Cold
Proper curing also requires that plaster be in a medium temperature
range. Usual recommendations range from 40F on the low side to 90F
on the high side (about 4C to about 32C). Too cold and there is
a risk that water in fresh plaster would freeze. As this is an expansive
process, cracking could occur. Cement hydration can be interrupted,
too. Too hot and there is a risk of drying—which, like freezing,
can also suspend cement hydration—or of accelerating the hydration
process to a point where strength development in the longer term
is negatively impacted.
Curing compounds are effective for concrete but are not used regularly
on plaster. These materials might interfere with subsequent coats
of plaster and might lead to discoloration of the stucco finish.
More information: Portland
Cement Plaster/Stucco Manual
(EB049)
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