MIL-HDBK-1110
7.4.1
Temperature. Most paints should be applied when the
ambient and surface temperature is between 45 degrees F and 90
degrees F. Lacquer coatings such as vinyls and chlorinated
rubbers, can be applied at temperatures as low as 35 degrees F.
There are other special coatings that can be applied at
temperatures below 32 degrees F but only in strict compliance
with manufacturer's instructions. Application of paints in hot
weather may also cause unacceptable films. For example, vinyls
may have excessive dry spray and latex paints may dry before
proper coalescence, resulting in mud-cracking. In all cases
painting must be done within the manufacturer's acceptable range.
Also, the temperature of the paint material should be at least as
high as the surface being painted. Paint should not be applied
when the temperature is expected to drop below 40 degrees F
before the paint has dried (except when allowed in the
manufacturer's instructions).
7.4.2
Humidity. Ensuring the proper relative humidity during
application and cure can be essential for good film performance.
However, different types of coatings require different relative
humidities. The coating manufacturer's technical data sheet
should be consulted. Some coatings cure by reacting with
moisture from the air (e.g., moisture-curing polyurethanes,
silicones, and inorganic zincs). These coatings require a
minimum humidity to cure. However, too high a humidity may cause
moisture-curing coatings to cure too quickly resulting in a
poorer film. In addition, too high a humidity may cause blushing
(whitish cast on surface of dry film) of some solvent-borne
coatings. Blushing is caused when the surface of a coating film
is cooled by evaporation of a solvent to such an extent that
water condenses on the still wet film. Excessive humidity may
also cause poor coalescence of latex coatings since the
coalescing agent may evaporate before enough water evaporates to
cause coalescence of the film.
7.4.3
Wind. Wind can cause a number of problems during spray
application. These include uncontrollable and undesirable
overspray and dry spray caused by too fast evaporation of the
solvents. The wind velocity at which these undesirable effects
occur depends upon the material being applied and the application
parameters. Wind can also blow dust and dirt onto a wet surface
which could lead to future paint breakdown.
7.4.4
Moisture. Paint should not be applied in rain, wind,
snow, fog, or mist, or when the surface temperature is less than
5 degrees F above the dew point. Water on the surface being
painted will prevent good adhesion.
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