Technical data

13773-001
51-20
Page 14 30 Nov 2000
H. Determining Ply Count
Ply count refers to the thickness of unfinished laminate (no surfacer, sealer or paint). To determine ply
count, measure the thickness of the damaged laminate (unpainted) in a relatively undamaged area.
Always verify and compare the measurement taken from the damaged laminate to the following ply
lay-up figures. A three-ply laminate without paint should measure approximately.030 inch.
WARNING:
Always use twice as many repair plies as the original lay-up in order to assure
that the original strength is achieved.
(a) Remove paint and primer by mechanical abrasion.
Note: Multi-action orbital type sanders or simple hand-sanding are the preferred
methods of paint removal. Fine-grit paper (120-grit or finer) should be
selected to minimize the potential for accidental damage.
CAUTION: Never use a grinder for removal of outer surface coats. A grinder will gouge
the surface creating more damage. Chemical strippers should not be used
as they may become trapped, damage the laminate or leave a residue. The
following figures show approximate areas which identify specific items of
repair.
Highly contoured or fine detail areas should always be sanded by hand.
Grinders, air files and other single-action tools tend to intensify pressure at
the edges and will rapidly remove paint and damage the underlying lami-
nate. Mechanical abrasion can also damage a laminate, and may be partic-
ularly damaging to certain joint designs (commonly leading edge and
fuselage joints).
(b) Using a micrometer, measure the thickness of undamaged and unpainted laminate next to
the area of damaged laminate.
(c) Note the micrometer reading. If the micrometer reads 0.010-inch, the laminate is only one
ply thick.
Note: A three-ply laminate without paint should measure approximately.030 inch.