in areas where pressurized cavities must be
Drying sealants set and cure by evaporation of the
solvent. The solvents in these sealants provide the
desired consistency for application. Consistency or
hardness may change when this type of sealant dries,
depending upon the amount of solvent it contains.
Shrinkage is a consideration when these sealants are
used. Shrinkage occurs upon drying. The degree of
shrinkage depends on the proportion of solvents.
Catalyst-cured sealants have advantages over
drying sealants. They are transformed from a fluid or
semifluid state into a solid mass by chemical reaction
of physical change rather than by evaporation of a
solvent. A chemical catalyst of accelerator is added
and thoroughly mixed just before sealant applications.
Heat may or may not be used to speed up the curing
process. When a catalyst is used, accurate
proportioning and thorough mixing of the two
components are very important to assure a complete
and even cure.
Application of Sealants
Application of sealants varies according to time
element, tools required, and the method of application.
However, the following restrictions apply to all sealant
Sealants should be used within the approximate
application time limits specified by the sealant
Sealants should not be applied to metal that is
colder than 70°F. Better bonding is obtained and the
applied sealant will have less tendency to flow out of
place while curing if the metal is warmed to a
temperature of 90°F to 100°F before the sealant is
Sealants should be discarded immediately when
they become too stiff to apply or work readily. Stiff or
partially cured sealants do not wet the surface to which
they are applied as well as fresh material. This causes
Sealants should not be used for close-fitting
(faying) surface applications unless they have just been
removed from refrigerated storage or freshly mixed.
Brushes, dipping, injection guns, spatulas, and
spray guns are the methods used to apply sealants.
Figure 4-35 shows (black areas) where sealant is
applied to protect some of the most corrosion-prone
areas on an F-14 aircraft. The sealant is applied by
using the spray, spatula, and brush methods.
Sealant MIL-S-81733, type III, is the sealant used
most extensively for spray application. If type III
sealant cannot be procured, sealant MIL-S-8802, class
A, may be used by thinning it to a sprayable
consistency by the addition of the correct solvent.
When an aircraft is pressure sealed, the sealing
materials should be applied as a continuous bead, film,
or fillet over the sealed area. Air bubbles, voids, metal
chips, or oily contamination prevent an effective seal.
Therefore, the success of the sealing operation
depends upon the cleanliness of the area and the
careful application of the sealant materials. There are
various methods of pressure-sealing joints and seams
in aircraft. The applicable SRM will specify the
method to be used in each application.
The sealing of a faying surface is done by brush.
The contacting surfaces are coated with the specified
sealant. Application of the sealant should be made
immediately before the parts are fastened together.
Careful planning of work and equipment are necessary
so faying surface seals on large assemblies may be
closed within the application time limit of the sealant.
Once the sealant has been applied, the parts must be
joined, the bolts torqued, and the rivets driven all
within the application time limit.
When insulating tape has been installed between
the faying surfaces to prevent contact of dissimilar
metals, pressure sealing should be done by fillet
sealing. In fillet sealing, the sealant is spread along the
seam with a sealant injection gun in about 3-foot
increments. Before proceeding to the next increment,
the applied portion of the fillet is worked in with a
sealant spatula or tool (fig. 4-36). This working of the
sealant is done to till in all voids in the seam and to
eliminate most air bubbles. The care used in working
out the air bubbles determines the leakfree service life
of the sealant. After the sealant has cured to a tackfree
condition, the fillet should be inspected for remaining
air bubbles. These air bubbles should be opened and
filled with sealant. When a heavy fillet is required, the
fillet should be applied in layers. The top layer should
fair with the metal.
Injection sealing is the pressure filling of openings
or voids with a sealant injection gun. The sealant is
forced into the opening until it emerges from the
opposite side. Voids and cavities are filled by starting
with the nozzle of the sealant injection gun at the
bottom of the space and tilling as the nozzle is