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If
it isn’t already on your pre-job checklist, add
this item: Check weather forecast. Why? Because the
weather conditions under which you apply exterior latex
paint can have a dramatic impact on performance...initial
and long term.
You
need, of course, to plan around predicted rain for
obvious reasons. And most painters are familiar with
the guidelines about painting after rain; namely,
the capability of latex paints to be applied before
the surface has dried 100%, and the need to apply
oil-based and alkyd coatings only after the surface
has thoroughly dried.
There
are, however, a number of other weather-related factors
that also must be taken into account, including temperature
and drying time.
TEMPERATURE
The binder in latex paint consists of microscopic particles of polymer
that need to fuse together when the paint dries to form a tough, continuous
film. Improper weather conditions can interfere with this process.
For
example, if the paint is applied at too low of a
temperature, the binder particles may become too
hard to fuse into a continuous, durable film. This
is because binder particles are thermoplastic, meaning
they tend to get harder at lower temperatures, and
vice versa.
This
is the main reason why paint manufacturers specify
a minimum application temperature (typically 50°F)
for latex products, and why it is essential that
you only apply the paint at or above the minimum
for that product. Ideally, you should not apply the
paint unless the temperature is predicted to stay
above that minimum for the next 36 hours.
Remember, too, that the minimum recommended temperature refers not just
to the air temperature, but also the temperature of the surface being
painted. This is particularly important for the north side of a home
or building, which gets less sun than other sides. The surfaces there
may be colder than the air temperature, particularly early in the day.
Failure
to follow these guidelines can cut years off the
life of a paint job. Incomplete film formation, for
example, can result in a paint film that fails by
cracking or flaking in just a few years or less. |
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Surfactant Leaching
Another situation influenced by the weather
conditions under which a latex paint is applied and
dries is a phenomenon known as surfactant leaching.
Also called streak staining, water-spotting,
and weeping, surfactant leaching refers to a concentration
of waterborne ingredients on the surface of a latex
paint that creates a blotchy appearance, often with
a tan or brownish cast.
While surfactant leaching usually
has no adverse effect on the long-term durability of
paint, it can temporarily affect the appearance of
a job before it naturally weathers off in a month or
two. Because it can impact a job, you should understand
its causes to help avoid its occurrence.
All exterior latex paints contain
some waterborne ingredients, such as glycols, surfactants
and thickeners. All of these eventually come out of
the paint film as it weathers, usually over the first
several weeks of exposure.
However, if the paint is applied in
cool, humid conditions that retard drying, a large
proportion of these ingredients can migrate to the
surface of the paint as it dries, or shortly thereafter.
There they typically appear as shiny streaks or blotches.
To minimize the possibility of surfactant
leaching, avoid painting late in the day, especially
in the spring and fall when cool, damp conditions are
expected in the evening or overnight. Also, avoid painting
just before it rains.
Remove
Carefully
While surfactant leaching will normally weather off, you may find that
immediate attention is required. If so, keep in mind that for the first
few days, the resistance properties of the paint will not yet be fully
developed. As a result, any immediate cleaning must be done carefully
and with plain water. Careful hand cleaning with a sponge is sometimes
effective.
Power washing in the early days of
the job may remove the paint and should not be used.
Power washing (use plain water) can be considered after
a week’s dry, but even then, proceed carefully
and test inconspicuous areas first to be sure the integrity
of the paint isn’t affected. Immediate repainting
is an uncertain solution, since any remaining surfactant
leaching may interfere with adhesion of the new paint.
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