Technical data

the cup from the moment the cup clears the liquid surface to
the point where the first break in the flow stream is noted.
This time is the viscosity in No. 2 Zahn seconds.
NOTE
The No. 2 Zahn Cup doesn’t work well for
high-solids coatings. The No. 4 Ford Cup is
preferable for these high-Solids coatings.
6.7.3.2 No. 4 Ford Cup. After the primer or paint
coating is properly mixed, pour the material into the cup
mounted in a stand while blocking the hole in the bottom of
the cup with a finger. Make sure the cup is completely filled,
and then scrape away any excess sliding a flat glass plate
across the rim of the cup. Leave the glass plate in place, and
remove the finger blocking the hole in the bottom of the cup.
Slide the plate horizontally to remove it from the top of the
cup. Using a stop watch, measure the time in seconds it takes
the material to flow out of the hole in the bottom of the cup
from the moment the plate is removed to the point where the
first break in the flow stream is noted. This time is the
viscosity in No. 4 Ford Cup seconds.
6.7.3.3 Adjustments. If the viscosity measured is not
within the specified range, thin the material per the thinning
instructions for the primer or paint coating listed in this
technical order, and remeasure the viscosity after cleaning the
cup with thinner. Repeat as necessary until the viscosity is
within the specified range.
6.8 BLUSHING.
Blushing of coatings is the result of moisture condensation
from the atmosphere within or on a drying organic film. It
occurs when the temperature of the work surface and/or the
coating material itself falls below the prevailing dew-point.
Always look for it in production coating operations whenever
high humidity conditions exist. It shows as a lighter-than-
normal or whitish discoloration or increased opaqueness of
film. It may occur to the extent that a surface powder results
which can be removed by a fingernail or by light rubbing.
(Rubbing will not cure the condition.) It may also be almost
imperceptible, and blush-inducing conditions may escape
notice on test panels sprayed prior to production coating due
to their temperature differing from that of the actual work
surface. Blushing most often occurs with quick-drying coat-
ings such as lacquers and only rarely with enamels. It is
detrimental in some degree to any coating material and will
affect adhesion of any overcoat. Any blushed areas must be
stripped and redone. Superficial blushing may occur at
borderline temperature and humidity conditions and subse-
quently disappear. This is not considered harmful, and may be
ignored.
NOTE
Slight blushing may sometimes be detected by
noting faint dissimilarities of appearance in the
film occurring over structural members underly-
ing the surface such as bulkheads, ribs, etc. Skin
surfaces in contact with internal structures may
be lower in temperature than the surrounding
skin, and this may be reflected and outlined in
blushing.
6.9 RESINS.
Natural resins are solid organic substances of vegetable or
animal origin. Synthetic resins are man-made substances
physically similar to natural resins. Rosin and shellac are
examples of natural resins. Synthetic resins have largely
taken over in modern coating formulations, because they can
be made to order to furnish desired characteristics. Coating
materials described in this technical manual are almost
exclusively based on synthetics. The following are examples
of resins and their special characteristics:
6.9.1 Alkyds. Alkyds are the backbone of modern
enamel in a great many combinations. Federal Specification
TT-E-489, is alkyd or modified alkyd enamel used by the Air
Force. Alkyd is characterized by toughness, flexibility and
durability, and is used in exterior and interior coatings for
utility and decorative purposes. It does not have good
chemical resistance.
6.9.2 Acrylics. Acrylics have outstanding light resis-
tance and outdoor weather durability, with moderate chemical
resistance. Used with nitrocellulose as a hardener, they have
better drying properties and increased hardness.
6.9.3 Vinyls. Vinyls have limited aircraft application,
but where construction materials, such as metal and wood
must be protected from high humidity, acidic or caustic
environments, vinyls are used. An example is Specification
MIL-P-15930 vinyl-zinc chromate primer.
6.9.4 Phenolics. Phenolic resins are used in varnishes
and enamels requiring extra hardness and abrasion resistance.
Specification A-A-1800 spar varnish is an example.
6.9.5 Silicones. Silicone resins are used primarily in
heat resistant coatings. They are not particularly hard sur-
faced and may craze. They are usually baked to cure.
6.9.6 Epoxies. Epoxies have outstanding adhesion and
chemical resistance. In combination with other resins, they
become baking enamels. For air drying types, catalysts are
TO 1-1-8
6-5