PREPARATION AND PAINTING OF AIRPLANE SKIN
The NESHAP regulations
required Boeing to change all components of the paint systems (solvent,
primer, topcoat, and application techniques). They also specified
that any cleaning solvents used before painting must have a vapor
pressure below 45 mm Hg. As a result, in conjunction with the new
high-solids primers and topcoats required, Boeing began using NESHAP-compliant
solvents that are less aggressive than the solvents used with conventional
paint systems (such as blends of methyl ethyl ketone [MEK] and toluene).
Surface preparation for
painting begins with using an alkaline cleaner to remove the green
temporary protective coating from the airplane skin. The airplane
then is wiped with solvent and abraded. To meet NESHAP regulations,
Boeing switched from the fast, strong, and effective 100 percent
MEK solvent to a slower evaporating solvent blend of methyl propyl
ketone and MEK.
Boeing also found an
alternative to abrading in the form of a recently developed acid
cleaner called Chemidize.
Water then is sprayed
over the airplane surface to clean it. Premature breaks in the water
film are a result of contamination. This is similar to the sheeting
action of water on a clean glass. Breaks in the water indicate that
the glass is dirty.
After the surface is
free of water breaks, the airplane is ready for the three-step finish
system: conversion coating, primer, and topcoat. The chromated conversion
coating, alodine 1000, allows the primer to adhere to the oxide
that is created on the airplane skin during conversion. Adhesion
occurs when the alodine reacts with the aluminum oxide and converts
it into a mixed chromate-aluminum oxide, which provides good adhesion
for the primer. After the excess alodine is rinsed off and the airplane
is dried, it is ready for primer and topcoat.
Primer is applied to
adhere to both the oxide and the topcoat to protect the aluminum
skin from corrosion. It is applied in one coat, typically 0.5 to
1.0 mil thick, and must cure in 2.5 hours. After curing, masking
tape is applied to define the decorative paint scheme for the airplane.
Three spray passes of
topcoat then are applied to achieve a coating 3 to 5 mils thick.
The paint hangar then is heated to accelerate the curing process.
The taping, painting, and curing processes may be repeated several
times to finish the paint scheme.