MIL-HDBK-1110
and the cure time can be altered. The equipment is cleaned by
purging with solvent. Because of the complicated nature of the
equipment, specialized operator training and skilled operators
are required. Initial and maintenance costs are also greater
than for other spray techniques.
7.5.4.6
Electrostatic Spray. In hand-held electrostatic spray
systems, a special protruding part of the gun is given a high,
negative voltage which places a negative charge on the spray
droplets as they come from the gun. The surface being painted is
grounded. This causes the paint droplets to be attracted to the
grounded surface to be painted. Because there is an electrical
attraction between the paint droplets and the object being
painted, a very high percentage of droplets lands on the surface.
That is, the transfer efficiency is high and there is minimal
overspray. Also, some droplets will be attracted to the edges
and the back of the surface, if they are accessible. This is
called the wraparound effect. Specially formulated paints are
required for electrostatic spraying. Also, painting is
restricted to use on conductive substrates, such as steel or
galvanized steel. Only one coat of paint may be applied to the
base metal by electrostatic spraying since a painted surface is
not conductive. Electrostatic spray is an ideal spraying method
for piping, fencing, channels, and cables because of the
wraparound effect and minimal overspray. However, because of
high voltage, special safety requirements must be met, including
grounding the power supply and the operator.
7.5.4.7
Powder Spraying. Powder coatings, usually epoxies, are
specially prepared polymeric coatings. They are applied to
preheated conductive surfaces, such as steel, by special
electrostatic spray equipment or in a fluidized bed. Once
applied, the coated component is heated to fuse the powder into a
continuous coating film. This technique is commonly used in shop
applications because heating can be done in an oven, there are no
volatile solvents to control and material that did not stick to
the surface can be collected and reused. Portable systems are
also available and can be used in special situations.
7.5.4.8
Thermal Spraying. Thermal spraying, sometimes called
metallizing, is a process in which finely divided metals are
deposited in a molten or nearly molten condition to form a
coating, usually on steel. Equipment and techniques are
available for flame or electric arc spraying of pure zinc, pure
aluminum, or an 85 percent zinc, 15 percent aluminum alloy. The
coating material is available in the form of a powder or wire,
with wire used more frequently. Once the metal becomes molten,
it is delivered to the surface with air or gas pressure. It
forms a porous coating that protects steel by cathodic protection
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