01 March 2005
finish (free from bubbles, wrinkles, filaments, or spray
dust). The running, lumping, or gathering of the film
into drops shall be avoided. Where practical, the dry
Components that are to be enclosed in airtight
film thickness shall be at least 0.002 inch.
cases should be allowed to air-dry at least
24 hours after varnish dries. Fumes may
accumulate to dangerous levels in the
containers and be ignited by sparks.
Varnish, ASTM D3955, shall be applied only
on clean, dry surfaces when the temperature
5-4.3.5. Special Precautions. When varnish is to be
is less than 100°F (38°C).
applied on certain types of equipment, special
precautions are required. The following is a list of
equipment and parts requiring special precautions:
All surfaces to be coated shall be free of dirt,
grease, and other foreign matter. Components
application of varnish will cause changes in some of the
that cannot be cleaned satisfactorily or that
circuit constants. These changes may be discernible
show evidence of corrosion should be replaced
only by electrical tests and measurements. A change in
by acceptable components.
alignment may be noted immediately after application
of the varnish. As the varnish dries and ages, further
a. Spraying. For larger equipment, a pressure pot
changes in circuit constants may take place. The greatest
spray gun with a tip regulated to give a wet spray is
change ordinarily will occur within 72 hours after
recommended. For small compact equipment, a pencil
treatment. The set should be completely realigned at
spray tip, regulated to give a narrow wet spray, is
the end of that period.
recommended. The varnish shall be applied in a wet
coat over all parts to prevent the formation of fuzz or
b. Coil Shields. When coil shields are removed and
filaments. A dry spray which forms spray dust shall not
replaced, they can be damaged and alter the tuning
be used. The equipment or individual assembly shall be
adjustments. If the damage is great, proper alignment is
sprayed from as many angles as necessary to assure
impossible. Extreme care must be exercised in removing
complete coverage with a wet coat. If more than one
and replacing coil shields.
coat of varnish is applied, sufficient drying time should
be allowed between each coat.
c. Trimmer Capacitors. Avoid spraying or brushing
varnish on the plates of trimmer capacitors. To minimize
b. Brushing. All parts which cannot be reached by
damage, all trimmer capacitors should be completely
spray shall be coated as completely as practicable with
covered during coating. If these capacitors fail to operate
a brush. A brush may also be used to cover small areas
satisfactorily after treatment, make a thorough inspection
not covered during the spraying process. On those
for deposits of varnish.
components requiring extensive masking, brush
application of the coating material may prove more
d. Tuning Slugs. Extreme care must be taken in
efficient than spray application.
removing and replacing tuning slugs. If varnish is
accidentally applied to a slug, remove it before replacing
c. Dipping. Subassemblies or components may be
coated by dipping, provided all requirements are met.
e. Discriminator Circuits. Careful adjustment of
5-4.3.4. Drying of Coated Equipment. Equipment
discriminator circuits after treatment is essential,
coated with varnish shall be dried by heating to 130°F
especially in the case of frequency-modulated
(54°C). Heating shall be gradual to prevent shrinking,
receivers. Discriminator circuits are more susceptible
to changes in circuit constants caused by varnish than
materials. The drying temperature should be maintained
for at least 1/2 hour but not longer than 3 hours. Drying
f. Tuned Circuits. Be especially careful during
may be done in a vented oven, vacuum oven, or with
masking to ensure that wires associated with tuned
Hot Air Gun, A-A-59435 (Volume IV or V, Chapter 3
circuits are not moved. Movement of such wires may
(shop use only)).
cause changes in circuit values.