Technical data

13773-001
20-10
Page 330 Nov 2000
Note: Thawed sealant must not be re-frozen. All sealants must extrude freely before
application to any surface. Adhesion may be lowered by condensation of moisture
if the sealant is not thawed to an extruded temperature of at least 60° F.
D. Application Time / Work Life
Application time refers to the time a mixed compound remains at a consistency suitable for application.
After this point, the sealant does not “wet” properly and may fail in adhesion (sealant “rolls up” ahead
of the sealing gun or “draws up” behind the gun). Application times shown in Figure 20-1 are based
upon an ambient temperature of 77° F (25° C) and a relative humidity of 50%. For fuel tank sealants,
every 20° F (10.6° C) rise in temperature halves application time and every 20° F (10.6° C) decline in
temperature doubles application time. High humidity during the mixing process will shorten application
time.
E. Tack-Free Time
Tack-free time is the time that a sealant requires to cure sufficiently to not stick to standard polyethyl-
ene film when the film is pressed into the sealant.
F. Cure Time
The time require to sufficiently cure applied sealant depends upon the ambient temperature and rela-
tive humidity. Always observe the manufacturer’s recommendations for cure. Low humidity will extend
the cure time. Optimum cure for most sealants will be obtained at 77° F (25° C) and 50% relative
humidity. A high state of cure is developed after 14 days at this temperature and humility. Cure may be
hastened by applying heat up to 130° F. For fuel tank sealants, every 20° F (10.6° C) rise in tempera-
ture halves cure time and every 20° F (10.6° C) decline in temperature doubles cure time. Fuel tank
sealant is considered to be sufficiently cured for pressure test when sealant is tack free, does not pit
when firmly pressed, and feels firm to the touch (Rex Durometer reading of approximately 30 A).
G. Types of Seals
(1) Fillet Seals
Fillet sealing is the application of a bead of extrudable sealant to seams, joints, and intersecting
planes or surfaces. Fillet seals are used in the integral fuel tank along the intersection of the
wing skin ribs, along the intersection of the wing skin and the spar and aft shear web, around
fasteners and fittings protruding into the tank. Fillet seals are also used around the firewall
perimeter, and other places where weather, fume, or dust intrusion must be prevented and to
seal wetted areas. Fillet seals are accomplished using extrudable sealants in an extrusion car-
tridge, tube, or spatula. Fillet seals may be tooled to improve appearance, fill voids and remove
bubbles. A satisfactory fillet seal is indicated when the fillet overlaps the seam by the required
dimension, is smooth in appearance, adheres to both sides of the seam, and is free of voids,
bubbles, and overlaps. Fillet seals may be tooled immediately after application to achieve the
seal.
(2) Faying Surface Seals
Faying surface seals are used between surfaces where an absolute seal cannot be obtained by
sealing around fasteners or by applying fillet seals. A faying surface seal around a fastener pre-
vents fuel or liquids from reaching the fastener thread or shaft. The faying surface seal does not
completely seal the fastener which must also be sealed. Faying surface seals are used during
the manufacture of the airplane to seal areas that are difficult or impossible to seal after assem-
bly. Faying surface seals may be applied by brushing on sealant or using and extrusion car-
tridge, tube, or spreading the sealant using a spatula. A satisfactory faying surface seal is
indicated when a small amount of sealant is extruded on both sides of the joint after assembly of
the faying surfaces.