Curing Is Critical
If the stucco is mixed properly and applied over a stable
substrate with properly nailed lath, then the conditions under
which it cures will determine the quality of the finished
If you want stucco that will last 100 years, pray for a
period of cool, damp days — temperatures in the 50s and
60s, no wind, and high humidity. Applying stucco in extremely
hot or cold conditions presents a real challenge: Stucco
applied in the cold needs to be heated up, and in extremely hot
conditions, it needs to be cooled down. Or said another way, if
the weather is hot and dry, you’ll want to slow the
curing, and if it is cold and damp, you’ll want to speed
When clients call and say "the finish coat of stucco is falling
off my house," I immediately suspect a dead-of-winter job.
Delaminating (unbonded) finished coats, which fall off the wall
in random-sized pieces like peanut brittle, should be inspected
for a telltale crystalline pattern on the back side of the
finish layer. That pattern suggests a finish coat that was
applied over a frozen (glazed) brown coat during winter
What typically happens is that sleet or blowing rain or snow
soaks the brown-coated walls. Unknowingly, the stucco
contractor applies the finish coat over a difficult-to-see
glaze of ice on the brown coat. Regardless of the ambient
temperature, if a finish coat is applied to the glazed walls,
the bond between the brown and finish coats will be lost.
The bottom line in working in the cold is don’t use
frozen material, don’t apply stucco to frozen or frosted
surfaces, and don’t mix materials or apply stucco when
the ambient temperature is less than 35°F.
If you can swing it, gutters and "shoot-outs" to direct the
melting snow and rain away from the walls make winter work much
easier. Scaffold planks should be pulled away from the walls
and tipped up so as not to serve as splash points for rain to
pelt the walls in horizontal bands and then freeze in the
Cement has an inherent degree of salinity, or natural salt
content. Adding an antifreeze, such as a calcium-based
accelerant, to the mixer in cold weather will delay freezing,
but it also heightens the salt content by magnitudes. This
greatly increases the likelihood of efflorescence — the
migration of salts to the surface — which leaves a
powdery white residue.
If cold weather is an issue, you can heat the water, warm
the sand with kerosene heaters, keep the raw materials covered
at night, and work in the sun, however meager it may be. You
can also cover or tent a house, but doing so is both time
consuming and expensive (Figure 5).
5. Covering stucco in cold or inclement weather
extends the curing period and protects against a
washout in case of rain.
Nevertheless, with large enough heaters, your working season
can be year-round, even in the northern states.
When it’s 75°F or higher in the sun and a hot wind is
blowing, it should be obvious that any mud applied to an
exposed surface will dry far more quickly than normal. And just
like concrete, stucco that dries too fast, or flash-dries, is
weaker than stucco that is allowed to cure slowly. In extremely
hot, dry, or windy weather, all your attention should be aimed
at slowing down the stucco’s drying and curing time and
protecting it from uneven and excessive evaporation.
First, keep the materials out of the direct sun. Sand holds
a lot of heat, as does water. If both are cold, the stucco will
dry slower. But don’t moisten the sand in an attempt to
cool it: Doing so will just confuse your mixing ratios.
Another simple way to avoid problems with the sun is to
start work early in the morning and plan to apply the stucco so
that the crew "chases the shadows" for the rest of the day
6. Working in the shadows is a good way to
prevent rapid drying on a hot day.
Working in shadows will slow down drying times. And be aware
that scaffolding planks may cast horizontal shadow lines on the
wall, causing those areas to dry more slowly. The stronger
stucco in these slow-cure areas will have a different rate of
absorption than the areas above and below the plank’s
shadow. Later, when the color coat is applied, it will be
"sucked" to the dissimilar areas unevenly, resulting in a
Misting and Draping
Your next defense against flash-drying is to keep the surface
of the new mud damp. This can be done on small areas and low
walls with a garden sprayer. On higher walls and over larger
areas, use a garden hose with a misting or fogging head (Figure
7. Spraying stucco in hot, dry, or windy weather
prevents excessive evaporation.
For the best results, the walls should be "hit" every hour
or so until the sun and wind are no longer a factor. Draping
new work with Visqueen or burlap also retards the evaporation
of water and prevents flash drying. And a soaker hose can be
run at the top of the wall to keep the burlap wet.
Another solution is to use a retardant like Sikafilm (Sika
Corp., 201 Polito Ave., Lyndhurst, NJ 07071; 800/933-7452), an
emulsified alcohol product that retards flash drying. It is
sprayed on the scratch or brown coats soon after application,
then washed off before the next coat is applied. Because the
product has an orange color, it is not recommended for finish
Depending on how fast the scratch coat is curing, it’s
possible to use the double-back method: The scratch and brown
coats are applied and cured as one system. The brown coat may
be applied as soon as the scratch coat is rigid enough to
receive the second coat without damage. This is a judgment
call. You run the risk of "stacking" too much mud too quickly,
thereby increasing the likelihood of checking in the brown
coat. If you double-back, misting becomes even more important
— I recommend doing it for 48 hours.
When it comes to the finish coat, you really have no choice
but to wait for a moderate day. Misting a finish coat creates
the potential for splotchy walls.