MIL-HDBK-1110
11.4.1.7 Fading. Fading of paint pigments occurs greatest in
the sunlight. Thus, there will be less fading of coatings under
eaves and other shaded areas. It also occurs more with synthetic
organic pigments than with naturally-occurring mineral pigments
(earth tones).
11.4.1.8 Discoloration. Discoloration is caused by exposure of
unstable pigments or resins to sunlight. Unstable resins like
polymerized linseed oil may yellow. The only prevention is to
use light-stable materials.
11.4.1.9 Pigment Overload. Pigment overload frequently results
in a mottled appearance or a poor quality film. It can occur
when attempting to tint a white paint to a deep tone. Latex
paints are particularly susceptible to this problem. By
specifying colors produced by the supplier, this problem can be
avoided.
11.4.1.10 Checking. Early checking may be caused by improper
formulation or application that causes the coating to shrink upon
curing. Excessive thickness or rapid curing may be responsible.
Aging will eventually cause checking of most coatings. It will
often occur when existing paints are topcoated with more rigid
finish coats that do not expand or contract as easily.
11.4.1.11 Dry Spray. Dry spray produces an uneven, pebbly finish
with holidays. The holidays can be verified on a metal substrate
with a holiday detector. It occurs most frequently when applying
coatings with fast evaporating solvents on warm days or when the
spray gun is held too far from the surface being painted.
11.4.1.12 Sagging. Sags may not permit complete curing of the
body of oil-based coatings and so may be soft below the surface.
Reduced film thickness in the areas where the sagging initiated
may be detected using a magnetic thickness gage on steel surfaces
and by using a Tooke gage on other surfaces.
11.4.1.13 Orange Peel. Orange peel is a defect of spray
application. It usually is widespread, when it occurs, and is
easily identified by its resemblance to the skin of an orange.
11.4.1.14 Wrinkling. Wrinkling occurs mostly with oil-based
paints that are applied so thickly on hot days that the surface
of the film cures rapidly to form a skin that does not permit
oxygen to reach the interior of the film to cure it. The coating
under the ridges is usually soft. Ridges generally occur in
parallel rows.
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