FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 1. Wood Structure | 5. Finishing Wood Structures | 1-69. Finishing of End Grain Surfaces
AC 43.13-1B
9/8/98
misalignment when mating the parts. It is
preferable to leave some unsealed areas rather
than risk weakening the joint by accidental
overlap of the sealer into the bonded areas.
Wherever possible, apply sealer to the margins
after the adhesive has cured. As an example,
the lower skin of a wing bay would be in-
stalled first, leaving access from above to ap-
ply sealer. All low spots (where moisture
would collect) are well sealed. The top skin
would be installed last, so that the only un-
sealed margins would be on upper surfaces
where moisture is least likely to collect.
e. An alternative to the previous para-
graph is to use an approved epoxy coating and
compatible epoxy adhesive. Apply the coat-
ing, allowing 1/4 inch margins as in the previ-
ous paragraph. After the coating has cured,
apply epoxy adhesive to joint surfaces, and
overlap the adhesive onto the sealer. Close
joint and clamp. The epoxy adhesive will
bond satisfactorily to the coating and ensure a
complete coverage of the wood surfaces. Use
only approved and compatible adhesives and
coatings for this method.
1-67. FINISHING OF INTERIOR SUR-
FACES. Finish repaired ribs, spars, interior of
plywood skin, and other internal members, in-
cluding areas of contact between metal and
wood, by applying one thinned coat (for pene-
tration into wood grain) of varnish or other ac-
ceptable finish, followed by two full coats.
Protect built-up box spars and similar closed
structures on the interior in the same way.
Where better protection is required, as on the
surfaces of wheel wells and the bottoms of
hulls below the floor boards, an additional coat
of aluminized sealer consisting of 12 to
16 ounces of aluminum paste per gallon of
sealer, may be applied.
1-68. FINISHING OF EXTERIOR SUR-
FACES. Exterior surfaces require more pro-
tection than interior areas due to the effects of
abrasion, weather, and sunlight. (See chap-
ter 2.) Tests have shown that the interior tem-
perature of wooden aircraft structures can
reach 185 �F when the aircraft is finished in a
dark color and parked outdoors on a hot, still
day. Exposure to prolonged high temperature
is detrimental to wood, adhesives, and finishes.
Wood loses approximately 25 percent of its
strength at 125 �F. For this reason, the me-
chanic should consider temperature effects
when selecting finish colors or looking for ar-
eas of likely deterioration. The lowest tem-
peratures are found when the aircraft is fin-
ished in white or very light colors, while
darker colors produce higher temperatures. A
general trend toward higher temperatures may
be seen when exterior colors are yellow, pink,
light blue, aluminum, purple, blue, light green,
orange, tan, red, green, brown, and black. A
lighter shade of a particular color helps to re-
duce temperatures.
1-69. FINISHING OF END GRAIN SUR-
FACES. End grain portions of wooden mem-
bers are much more absorbent than side grain.
Because of this extreme vulnerability to mois-
ture entry, it is necessary to take extra precau-
tions to seal end grain.
a. Apply at least one thinned coat of ac-
ceptable sealer to ensure maximum penetra-
tion, and then follow with as many full
strength coats as necessary to achieve a
smooth, glossy coating. Depending on the
type wood to be finished, two to four full coats
will be required. A final coat of aluminized
varnish may be applied to end grain surfaces.
If the surfaces are to be finished with dope or
lacquer, a dope-proof sealer, similar to Federal
Specification TT-V-109, or epoxy sealer
should be used.
b. Exposed end grain includes such sur-
faces as spar butts, skin edges, areas around
vent holes, inspection holes, fittings, and ex-
posed scarfed or tapered surfaces.
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