MIL-HDBK-1110
of towers. While lighting may be less expensive in initial
construction and maintenance, an unprotected bare zinc surface
will erode and require more expensive repairs than a bare
surface. Further, some studies have shown that the lifetime of
the zinc plus organic coating system is significantly greater
than the sum of just the zinc coating and of an organic coating.
While painting automatically brings maintenance problems, these
are normally much less than those occurring to unpainted towers.
The orange and white colors required by the FAA are available in
aliphatic polyurethane, alkyd, and latex formulations.
5.7.1
New Towers. Today, new tower components are usually
built with galvanized structural steel or steel thermally sprayed
with zinc metal, if too large to be placed in a dipping tank.
5.7.1.1
New Galvanized Steel Towers. Galvanizing applications
for steel tower components are typically heavy (e.g., 4 to 7 mils
of zinc) and accomplished by hot dipping. Whether thermally
sprayed or hot dipped, the zinc coating can provide several years
of protection by itself. However, it will subsequently be
necessary to apply a paint system to extend this corrosion
protection, after the zinc is consumed. Because quality painting
of towers after erection is both difficult and expensive, it is
always best to apply organic coatings beforehand, preferably in a
shop setting. Surface preparation and painting of tower
components in a shop can be accomplished under controlled
conditions to provide optimum protection of the metal. Shop
cleaning of zinc-coated surfaces is normally limited to detergent
washing to remove loose contaminants and/or solvent cleaning
(SSPC SP 1) to remove grease or oil. Sometimes, a thin film of
grease or oil is applied at the factory to protect galvanizing
from corrosion during exterior storage. Also, new galvanizing is
sometimes treated with chromate corrosion inhibitors for
corrosion protection during storage. Such treatment should
specifically be excluded in specifications for galvanized steel
components to be coated.
Galvanized steel components are best protected with one coat each
of epoxy-polyamide (e.g., MIL-P-24441 Formula 150) and aliphatic
polyurethane (e.g., MIL-C-85285) coatings as described above. If
a delay of over 4 days occurs before topcoating, the finish coat
of polyurethane may not adhere because of the solvent resistance
of the nearly fully cured epoxy coat. A thin (2-mil wet film
thickness) film of the epoxy primer applied and allowed to cure
to a tacky finish (e.g., 4 hours) will provide a suitable surface
for the polyurethane finish coat. Epoxy and urethane coatings
must have at least a 6-hour pot life for practical coating of
towers in place. Oil-based paints (including oil/alkyds) are not
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