sealant on both sides. Split grommets should have
Careful planning is necessary to close faying
sealant brushed into the split prior to installation. After
surface seals on large assemblies within the application
installation, fillets should be applied to both the base of
time limit of the sealant. Once the sealant has been
the grommet and the protruding tube in the pressure
applied, the parts must be joined, the required number
side.
of bolts must be torqued, and all the rivets driven within
this time limit.
Sealing Compound MIL-S-8802
When insulating tape has been installed between
the faying surfaces to prevent dissimilar metals
This temperature-resistant, two-component,
contacts, pressure sealing should be accomplished by
synthetic rubber compound is used for sealing and
fillet sealing. Fillet sealing is the spreading of sealant
repairing fuel tanks and fuel cell cavities. This
along the seam with a sealant injection gun. The sealant
compound is designed for an operating environment
should be spread in approximately 3-foot increments.
that may vary between -65F and +250F. It is produced
Before you proceed to the next increment, the applied
in the following classifications:
portion of the fillet should be worked with a sealant
spatula or tool. See figure 5-44. This working of the
Class A. Sealing material suitable for brush
application,
sealant fills the voids in the seam and eliminates air
bubbles. The leak-free service life of the sealant is
Class B. Sealing material suitable for application
determined by the thoroughness and care you use in
by extrusion gun and spatula
working out the air bubbles.
Class C. Sealing material suitable for faying
After the sealant has cured to a tack-free condition,
surface sealing
the fillet should be inspected for any remaining air
Dash numbers after the classification code indicate
bubbles. Such air bubbles should be opened and filled
the allowed application time in hours before the curing
with sealant.
cycle will have progressed to the point where it is no
When a heavy fillet is required, it should be applied
longer feasible to apply that particular batch of sealant.
in layers. The top layer should fair with the metal.
Class A dash numbers are -1/2 and -2. Class B dash
numbers are -1/2, -2, and -4. Class C dash numbers are
Injection sealing is the pressure filling of openings
-20 and -80 (8 hours of application time with the
or voids with a sealant injection gun. Joggles should be
remaining time allowed for working the material).
filled by forcing sealant into the opening until it
emerges from the opposite side. Voids and cavities are
Example: Class A-2 designates a brushable
filled by starting with the nozzle of the sealant injection
material having an application time of 2 hours. Class
gun at the bottom of the space and filling as the nozzle
B-1/2 designates an extrusion gun material having an
is withdrawn.
application time of 1/2 hour. Class C-20 designates a
faying surface sealant with an application time of 8
NOTE: A joggle is a joint between two pieces of
hours and a working life of 20 hours.
material formed by a notch and a fitted projection.
Rivets, rivnuts, screws, and small bolts should have
Sealing Compound MIL-S-81733
a brush coat of sealant over the protruding portion on
the pressure side. Washers should have a brush coat of
This accelerated, room temperature, curing
synthetic rubber compound is used in sealing metal
components on weapons and aircraft systems for
protection against corrosion. This sealant contains
magnesium chromate as a corrosion inhibitor. The
classification of this sealant compound is of the
following types:
Type I. For brush or dip application
Type II. For extrusion application, gun or spatula
Type III. For spray gun application
Dash numbers after the type code are used to
designate the maximum application time in hours. Type
Figure 5-44.--Appling sealant.
5-39
