FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 2. Fabric Covering | 2. Application of Dope | 2-20. General
9/8/98
SECTION 2. APPLICATION OF DOPE
AC 43.13-1B
2-20. GENERAL. Nitrate dope and bu
tyrate dope are manufactured by treating cel
lulose, derived from wood pulp or cotton linter
with select acids, then dissolving in a blend of
solvents and adding plasticizers for flexibility.
After a brush or spray application on fabric,
the film develops tension and strength as the
solvents evaporate. The tension and strength
will increase in proportion to the total film
thickness. The fabric functions as a film for
mer and carries no load until a crack develops
in the dope film. An excessively-thick dope
film will develop too much tension and may
warp or damage a light airframe.
a. Viscosity adjustments for brush or
spray applications may also vary between
brands. When the viscosity adjustment ratio is
not provided or is unclear, the product manu
facturer should be contacted for detailed in
structions. If instructions cannot be obtained
or the source of the dope is unknown, the dope
should be considered suspect and its use is not
advised.
not necessary to use nitrate dope for the first
application and butyrate dope for all other ap
plications. The presence of naphtha in nitrate
dope, manufactured in accordance with can
celed Mil Specs formulas, causes nitrate dope
to be incompatible with butyrate dope; there
fore, nitrate dope should not be applied over
butyrate dope for repairs or refinishing; how
ever, butyrate dope may be applied over nitrate
dope.
NOTE: Nitrate or butyrate dope
thinners and retarders should not be
substituted for each other, nor should
automotive coating-type thinners be
used.
d. Clear dope produces the most tension
and strength. Aluminum-pigmented dope will
weigh slightly more than clear dope and de
velop less tension and strength for the same
film thickness. Pigmented color finishes will
produce the least tension and strength due to
the higher ratio of plasticizers.
b. Dope, which has been stored for an
extended period of time or under adverse con
ditions, should be suspected of becoming
acidic and should be tested before being used
on cotton or linen fabric. In some cases, fresh
production dope has also been found to have a
high acid content and will begin to deteriorate
cotton or linen in a period of a few months.
The acid content of nitrate or butyrate dope
should not exceed 0.06 percent, calculated as
acidic acid. An acidity test can be performed
by most testing laboratories if high acid con
tent is suspected.
c. Butyrate dope is superior to nitrate
dope in weather exposure tests. However, ni
trate dope provides better adhesion to natural
fiber than butyrate dope. The adhesion of bu
tyrate dope to natural fibers is adequate; it is
e. During the coating-buildup procedure,
solvents released from each succeeding coat
will penetrate and be absorbed into the previ
ous dope film, temporarily releasing the ten
sion and increasing the drying time between
coats as the dope film becomes thicker. If
elapsed time between coats exceeds several
weeks at temperatures above 70 F, it is rec
ommended that several spray coats of an ap
propriate dope thinner or dope with retarder
and/or rejuvenator added be applied to the
lightly-sanded, dried dope film to open the sur
face and provide cohesion for the next coat.
This will reduce the possibility of surface
cracks caused by dissimilar tension between
the old and new dope film.
f. All dope coats through the final finish
may be applied with a brush; however, brush
Par 2-20
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