Three-Coat Stucco, continued
The Scratch Coat
The scratch coat is the first coat of stucco, and it serves as
the foundation for the next two coats. It should be harder than
the brown coat, which we achieve by using a richer mix -- about
one part cement to two to four parts sand, or 21 to 24 shovels
of sand per sack of cement. After the scratch coat is applied,
the plasterer scores the surface with a special scarifier rake
or a scratching trowel. This creates extra surface area and a
better bond for the brown coat. In the first few hours after
the scratch coat is installed, the cement is very fragile and
should not be disturbed while it slowly gains strength. Because
the scratch coat is thin, the moisture evaporates quickly. If
it dries out too quickly, it will be weak as a result of
incomplete hydration. To prevent this, you'll need to mist the
surface with water.
Verify the hardness of the scratch coat before applying the
brown coat. To test for hardness, we drag a nail over the wall.
If the nail does not dig in but leaves a white line, the stucco
should be hard enough. If the nail does dig in, then the
scratch coat is not hard enough and should be wetted so the
cement can hydrate more thoroughly.
The scratch coat should be no thinner than the brown coat,
and firm enough to resist pressure from your hand when you lean
against it. At least 95% of the lath should be covered by this
coat. If there are any "dropouts," they should patched or
filled that same day.
The Brown Coat
This is a leveling coat that provides the flat surface for the
finished wall. The brown coat also adds strength and thickness,
and is in large part what determines the quality of the finish.
It's a little sandier, at one part cement to three to five
parts sand, or about 25 to 30 shovels of sand per sack of
cement. The increased sand helps reduce the number of shrinkage
The application of the brown coat also reintroduces water
into the scratch coat; because of the additional coverage,
moisture will be better retained in the base coat, resulting in
even more thorough hydration and a good, dense base.
You should not apply the brown coat until the scratch coat
is hard. Floating the brown coat makes for a denser product --
we use hard rubber floats. Wood floats will provide a
straighter surface, but they are rarely used nowadays.
The Finish Coat
This is what provides the final texture and color. Premixed
finish coat material usually works fine, except I would avoid
dark colors, especially reds. Because stucco colors are mineral
pigments, dark colors are prone to spottiness and, if not well
blended, will not match the color sample.
The only limits on the texture and appearance of the finish
are the imagination and skill of the applicator. If you are new
to using stucco and are planning to hire a sub, visit his
previous jobs. Watch for uneven textures and appearance,
discoloration, poor color matching, and irregular
Weather and Curing
Curing is an important part of a top-quality stucco job, and
weather has a lot to do with curing. Weather conditions with
high humidity, such as drizzly rain or heavy fog, are favorable
to proper hydration of cement. If weather conditions are drier,
you may need to gently spray the wall with a hose to ensure
enough moisture for complete hydration and to avoid cracking.
Stucco must be moist-cured for at least 48 hours, unless
weather conditions are favorable (above roughly 80% RH, you
don't need to moist-cure).
According to code, there should be at least 48 hours of
curing time after the first coat is applied, and seven days
after the second coat is applied if there is to be a third
coat. The critical consideration is that the scratch coat must
As the first coat of stucco dries, it will lighten in color
(Figure 6). This will be especially visible on the sunny side
of the house first. When this happens you must fog or mist the
stucco with water. Once it hardens (the next day), you don't
have to be as careful -- you can squirt water on the wall.
6. As the scratch coat dries it will change
color. It is important to make sure there is enough
moisture available for complete hydration of the cement
by misting the wall with water.
You should also protect stucco from freezing for at least 24
hours after it has set; you also must not use frozen materials
or apply materials to a frozen base. The bottom line is this:
If the weather isn't right for stucco, you should wait until it
During application, part of the stucco will end up somewhere
besides on the wall -- you can count on it. The responsibility
for finish protection and cleanup should be carefully described
in the scope of work. Make sure you know who is responsible for
cleaning up the site and hauling away excess material and
waste. Site protection, including cleanout areas, should be
addressed before you start the job. Protection of glass, doors,
window and door tracks, gutters, and landscaping from stucco
slag is imperative (Figure 7) because, if left in place,
finished surfaces can be irreparably damaged. For example,
stucco can permanently etch glass in a matter of days. You'll
want to clean up right away -- it's always easier to clean up
the cement droppings before they're hardened.
7. Finish protection is absolutely essential.
Make sure you understand who is responsible for this
before the job begins.
Small shrinkage cracks in stucco should be anticipated; what
is acceptable depends on customer expectations. Following
proper curing practices and using good mixes with the right
amount of water will reduce cracking.
Cracks in the scratch coat get filled by the brown coat. If
you see larger cracks in the brown coat, however, it's best to
fill them before proceeding with the color coat.
Hairline cracks (anything smaller than 1/16 inch) in the
color coat do not allow water intrusion, so it really serves no
useful purpose to patch them. The patch will inevitably be a
different color and will only detract from the beauty of the
Structural cracks -- cracks that go all the way to the back
plane of the plaster -- are another story. Although the
weather-resistive paper should prevent water from getting to
the framing, it's best to patch any crack that goes all the way
through the scratch coat. As mentioned earlier, the best cure
for these cracks is prevention: Make sure the foundation
doesn't settle and use dry, straight framing materials.
Ron Webberowns and operates Prime Plastering in
Norco, Calif. He has worked as a plasterer for more than 30