Sequoia Aircraft Corporation F.8L Falco, Construction Manual

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F.8L Falco Construction Manual 9–1 Revision 4, March 1, 2002
Chapter 9
Moisture Protection of Wood
Introduction
All of the interior and exterior wood surfaces must be covered with a coating to prevent the
absorption of moisture. This chapter will deal only with the interior surfaces.
A complete discussion can be found in the “Finishing Wood in Aircraft” section of ANC-19.
This publication describes the methods used on wood aircraft at the time of its publication
(1951). The technology of finishing wood has been greatly advanced since then.
Requirements of Finishes
The primary objective of interior finishes is to afford protection of the wood against serious
change in moisture content when exposed for a limited time to damp air or to water that gains
access to closed spaces by condensation or by penetration of rain, mist or fog through joints, vent
holes or imperfections in the covering. Coatings on contact areas between wood and metal
protect the metal against corrosion from moisture in the wood.
Interior finishes must retain their protection for the life of the aircraft and throughout the great
range in temperature to which the craft is subjected in service. They need not be capable of
withstanding exposure to the weather, including sunshine, for any such length of time.
The primary objectives of exterior finishes are protection of the wood against weathering,
sufficient smoothness of surface to minimize skin resistance during flight, suitable appearance and
enough durability to retain these properties for several years of full exposure to the weather at all
altitudes of flight even in the most severe climates. The finish should be easy to keep clean and
should wear in such a way that it can be renewed when necessary with minimum increase in the
weight of the coating.
Protective Power of Wood Finishes
When unprotected wood is exposed alternately to dampness and dryness, such as to rain and
sunshine, the portions of the wood near the surface change in moisture content more rapidly and
more widely than do the interior portions. Such unequal distribution of moisture within pieces of
wood sets up internal stresses that are responsible for such processes of weathering as grain-raising,
cupping, warping, checking and softening and disintegration of the surface. Protective finishes
guard against weathering by retarding the rate at which moisture passes though them, either into
or out of the wood surface, to such an extent that a reasonably uniform distribution of the
moisture within the pieces of wood is always maintained and internal stresses are thereby avoided.
A finish entirely impermeable to moisture has not yet been found. Moreover, no finish alters the
fiber-saturation point or swelling coefficient of wood. Many tests have proved that all finishes,
including multiple coats of polyurethane and epoxy, do not stop the moisture content of the air
from changing the moisture content of the wood. There are many finishes which will keep water
in a liquid state from entering the wood, but water vapor in the air will penetrate all known
finishes.
The 1950’s Techniques
The techniques and materials included here are a summary of the suggested procedures from
ANC-19 .