Sealant should be used within the application
Pliable Sealants
time limits specified by the sealant manufacturer.
Pliable sealants are referred to as one-part sealants
Sealant should not be applied to metal that is
and are supplied "ready for use" as packaged. They are
colder than 70F. Better adhesion is obtained and the
solids and change very little during or after application.
applied sealant will have less tendency to flow while
Solvent is not used with pliable sealants. Therefore,
curing if the metal is warmed to a temperature between
drying is not necessary. Except for normal aging, they
90F and 100F before the sealant is applied.
remain virtually the same as when they were packaged.
Sealant should be discarded immediately when
They easily adhere to metal, glass, and plastic surfaces.
Pliable sealants are used around access panels and
it becomes too stiff to apply or work. Stiff or partially
doors and in areas where pressurization cavities must
cured sealant will not wet the surface to which it is to be
be maintained.
applied as well as fresh material and, consequently, will
not have satisfactory adhesion.
Drying Sealants
Sealant should not be used for faying surface
applications unless it has just been removed from
Drying sealants set and cure by evaporation of the
refrigerated storage or freshly mixed.
solvent. Solvents are used in these sealants to provide
While the use of sealants on aircraft surfaces has
the desired application consistency. Consistency or
greatly increased over the past few years, application
hardness may change when this type of sealant dries,
methods have been mostly through the use of brushes,
depending on the amount of solvent it contains.
dipping, injection guns, and spatulas. The spraying of
Shrinkage during the drying process is an important
sealants is a recent development. MIL-S-81733 sealant,
consideration. The degree of shrinkage also depends
type III, is extensively used for spray application. If
upon the amount of solvent it contains.
type III sealant cannot be procured, MIL-S-8802
sealant, class A, may be used by thinning it to a
Curing Sealants
sprayable consistency by the addition of an appropriate
solvent.
Catalyst-cured sealants have an advantage over
When you are pressure sealing an aircraft, the
drying sealants because they are transformed from a
sealing materials should be applied to produce a
fluid or semifluid state into a solid by chemical reaction
continuous bead, film, or fillet over the sealed area. Air
rather than by evaporation of a solvent. A chemical
bubbles, voids, metal chips, or oily contamination will
catalyst or accelerator is added and mixed just prior to
prevent an effective seal. Therefore, the success of the
sealant applications. Heat may be employed to speed up
sealing operation depends upon the cleanliness of the
the curing process. When you use a catalyst, you should
area and the careful application of the sealant materials.
accurately measure and thoroughly mix the two
There are various methods of pressure sealing the joints
components to ensure a complete and even cure.
and seams in aircraft. The applicable structural repair
manual will specify the method to be used in each
APPLICATION OF SEALANTS
application.
The sealing of a faying surface is accomplished by
The application of sealants varies according to
brush coating the contacting surfaces with the specified
time, tools required, and the application method.
sealant. The sealant should be applied immediately
However, the following restrictions apply to all sealant
before fastening the parts together.
applications:
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