(1) Aluminum bare metal areas will be alodine
treated per MIL-C-5541, Classes 1 or 2, prior to applying
the first primer.
(2) Bare magnesium metal areas will be dichromate
treated per MIL-M-3171, Type 6, prior to applying the
Paint will not bond to the brown powdery
magnesium or aluminum, nor will paint
bond to treated aluminum surfaces which
are too dark.
(3) Steel substrates will be treated with zinc
phosphate per TT-C-490, Type I, prior to applying the
(4) All missing or deteriorated skin sealant will be
replaced using MIL-S-81733, Type II, Class B, sealing
(5) Copper and copper base alloys are quite
resistant to atmospheric corrosion. Protective coatings
are seldom required because of the inherent resistance
of the metal. Corrosion attack on such components will
usually be limited to staining and tarnish. Generally
such change in surface conditions is not dangerous and
should ordinarily have no effect on the function of the
part. See TM 55-1500-344-23. Paint finishes may be
applied for decorative purposes.
g. Mating Surfaces. The use of primers on similar
and dissimilar metal mating surfaces and on attaching
parts is presented in the following paragraphs.
(1) Application on similar metals. All seams in
which the mating (faying) surfaces are similar metals
will be protected by applying two coats of primer to each
surface being joined. Welded faying surfaces shall not
be primed prior to assembly. Faying surfaces that are to
be adhesively bonded will be surface treated and
processed as specified in the approved bonding
procedures prescribed for the assemblies concerned. In
addition to the required primer coatings, faying surfaces
of magnesium alloys will be filled with an approved
(2) Application on dissimilar metals. Each surface
to be joined will receive a minimum to two coats of the
appropriate primer, and, in addition, the following
precautions will be taken:
(a) Where magnesium is one of the metals of
dissimilar metal faying surfaces, the metals will be
separated by use of an approved barrier tape. The tape
will extend not less than 1/4 inch beyond the joint edges
to prevent moisture from bridging between the dissimilar
metals. Sealing compounds conforming to MIL-S-81733
be used as moisture barriers in lieu of tape and when
used, the sealing compound will be applied between the
surfaces and squeezed out of all boundaries and the
excess removed in a manner that will fillet all edges.
The fillet width will be not less than 1/4 inch. Joint areas
which would retain water will be filled with a sealing
compound conforming to MIL-S-81733 or MIL-S-7124.
When the use of a barrier tape or sealing compound is
impractical because of mechanical or other factors, a
primed 5356 aluminum alloy shim extending 1/4 inch
will be used in lieu of the tape or sealing compound.
(b) Butt joints consisting of a riveted magnesium
sheet and a riveted dissimilar metal sheet will be
protected by grooving the seam to a width of not less
than 1/8 inch and filling with sealing compound
conforming with MIL-S-81733. The depth of the groove
will be sufficient to retain the sealing compound
conforming to MIL-S-81733, which will be subsequently
applied and smoothed flush with the surfaces of
adjacent dissimilar metals.
(c) Butt joints consisting of a magnesium sheet
and a dissimilar metal sheet, neither of which is secured
by mechanical means other than riveting, will be
protected by use of an approved adhesive barrier tape
or a primed 5052 aluminum alloy foil which covers the
cut ends of either or both dissimilar metal sheets and
extends not less than 1/4 inch beyond the joint edge.
(d) Organic adhesive barrier tapes or metal foils
will not be used on exterior surfaces.
(3) Attaching parts. Parts, such as nuts, bushings,
spacers, washers, rivets, high shear rivets, screws, self-
tapping screws, sleeves for shakeproof fastener studs,
self-locking nuts, speed nuts, clamps, and the like, do
not need to be painted in detail except when dissimilar
metals or wood contacts are involved in the parts being
joined. Such parts will receive a coat of epoxy primer,
MIL-P-23377 (wet or dry, at installation). Rivets (5056)
in magnesium alloy need not be installed with epoxy
primer. For magnesium dissimilar combinations, follow
the general requirements. Close tolerance bolts passing
installation with a zinc chromate compound conforming
to MIL-C-11796, Class 3, MIL- C-16173, Grade I, or a
wet epoxy primer in accordance with MIL-P-23377.
Aluminum alloy washers (5356) of suitable