MIL-HDBK-1110
recommended because of the inherent incompatibility of oil-based
paints with the alkaline surface of galvanizing. Premature
failure by peeling is predictable.
A two-coat latex system (e.g., 1-1/2 mils dry film
thickness each of MIL-P-28577 primer and MIL-P-28578 topcoat or
SSPC PAINT 24) can also be used on galvanizing, but the
protection and gloss and color retention may not be quite as good
as with the epoxy/polyurethane system. The corrosivity of the
exposure environment should be considered when choosing between
the two systems.
5.7.1.2
New Thermally Sprayed Steel Towers. Thermally sprayed
zinc is relatively porous and protects steel by cathodic
protection. It should be sealed to provide maximum protection.
Application of epoxy polyamide MIL-P-24441, Formula 150 thinned
50/50 has been very effective in sealing of thermally sprayed
ship components. Where restrictions on the solvent (VOC) content
prevail, sealing can be accomplished with a mist coat. Sealing
should be followed with a full coat of Formula 150 applied at the
usual 3-mil dry film thickness and a finish coat of aliphatic
polyurethane (e.g., 2 mils dry film thickness of MIL-C-85285).
Some private companies have successfully coated
thermally sprayed steel components with a single, heavy (e.g., 6
to 8 mils dry film thickness) coat of commercially available
aluminum-filled epoxy mastic. Such a product is not covered by
Government or industry specifications.
5.7.1.3
New Steel Towers. If new steel tower legs are not
galvanized or thermally sprayed with a zinc coating, use of an
inorganic zinc coating should be considered if the coating can be
applied in the shop. A controlled shop environment can provide
the proper conditions for obtaining a very corrosion-protective
inorganic-zinc coating. These coatings (e.g., SSPC SP 5 surface
preparation and MIL-P-24648, inorganic zinc) have been shown to
provide long-term service with minimal maintenance requirements.
If the coating must be applied in the field, an organic zinc-rich
coating is probably preferred since they are more forgiving of
surface preparation lapses and can be applied and cured over a
wider range of environmental conditions. For either system, an
intermediate coat of epoxy polyamide (e.g., MIL-P-24441, Formula
150) and a finish coat of aliphatic polyurethane (e.g.,
MIL-C-85285) can complete the system.
5.7.2
Existing Towers. It is best to repair damaged tower
coatings on existing towers on a regular schedule before the
damage becomes significant. To repair or topcoat existing
coatings, it is necessary to know the generic type of the present
coating. The same or another compatible coating must be used.
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