Surface Coating of Miscellaneous Metal Parts
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Applications, Curing, Equipment and Emissions Sources
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Coatings Application
 Following surface preparation, paints and coatings are applied
to substrates using a variety of methods, including:
- Dip coating, in which parts are dipped into tanks of paint and
the excess paint is allowed to drain off;
- Roller, in which paint is rolled onto a flat part;
- Curtain coating;
- Flow coating;
- Electrodeposition, in which a part is coated by making it anodic
(+) or cathodic (-) in a bath that is generally an aqueous emulsion
of the coating; and
- Various spray processes, in which paint is sprayed from a gun
onto a part.
 Coatings are usually applied in a number of coats, starting
with a prime coat followed by subsequent coats (basecoats and
topcoats) and a finishing coat (clearcoats). Given the different
types of coatings necessary to ensure adequate protection and
performance, coatings should always be considered as a system.
Curing
 Once a coating has been applied, a curing process takes place
that converts it into a hard, tough, and adherent film. Coatings
cure by chemical reaction or polymerization of the resins
(i.e., crosslinking). Mechanisms for initiating curing generally
include ambient temperature oxidation, chemical reaction with
another component (two-component coating systems) or baking
in an oven. Radiation is an additional curing mechanism.
Equipment Cleaning
 The final stage of any coating operation is the cleaning of
equipment, such as spray guns and hoses. Traditionally this
generally involves flushing solvent through the coating system.
Spray guns can be cleaned manually or with several different
types of gun cleaning systems specially designed for this
purpose. Cleaning of equipment results in VOC/HAP emissions.
Solvent emissions from gun cleaning equipment occur both during
actual cleaning operations ("active losses") and
during standby ("passive losses") periods.
Sources of Emissions from Coating Operations
 Traditionally, each step in the coating process generates waste
and emissions. The initial mixing of coatings can be a source
of emissions. Inefficient paint transfer can be the largest
source of waste and VOC/HAP emissions from paint and coating
processes. Paint used, but not applied to the surface being
coated (e.g., paint overspray), generally becomes waste.
Evaporation of organic solvents is an important source of air
emissions. During coating applications, solvents that are present
in conventional paint formulations evaporate and release VOCs/HAPs
into the air. Emissions occur during initial coating, as well
as each time a surface is recoated during the life of the object
or structure. In addition, solvents used to thin paint and to
prepare surfaces for coating can be sources of emissions. The
curing process can also add to emissions.
Cleaning of equipment is a major source of waste generation.
Generally, all paint-application equipment must be cleaned after
each use to prevent dry paint residue and avoid contaminating
batch processes. In addition, brushes and rollers must be cleaned
after each use to remain pliable.
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