Is this the type of corrosion that showed up under the paint of Cessnas a few years ago due to lack of proper prep? As I remember the first you would notice were "bubbles" in the paint.
From AC 43.13-1B
6-16. FILIFORM CORROSION. Filiform
corrosion is a special form of oxygen concentration
cell which occurs on metal surfaces
having an organic coating system. It is recognized
by its characteristic worm-like trace of
corrosion products beneath the paint film.
(See figure 6-8.) Polyurethane finishes are especially
susceptible to filiform corrosion. Filiform
occurs when the relative humidity of the
air is between 78 and 90 percent and the surface
is slightly acidic. This corrosion usually
attacks steel and aluminum surfaces. The
traces never cross on steel, but they will cross
under one another on aluminum which makes
the damage deeper and more severe for aluminum.
If the corrosion is not removed, the area
treated, and a protective finish applied, the corrosion
can lead to inter-granular corrosion, especially
around fasteners and at seams. Filiform
corrosion can be removed using glass
bead blasting material with portable abrasive
blasting equipment or sanding. Filiform corrosion
can be prevented by storing aircraft in an
environment with a relative humidity below
70 percent, using coating systems having a low
rate of diffusion for oxygen and water vapors,
and by washing the aircraft to remove acidic
contaminants from the surface.
