K 2500 Truck 4WD V8-379 6.2L DSL VIN C FI (1989)
Paint: All Technical Service Bulletins
Paint - HVLP Painting Information
GMC NUMBER:
91-10-109
GROUP:
10 Body Section
DATE:
June, 1991
CORPORATE
NUMBER:
161701R
SUBJECT:
VOLATILE ORGANIC COMPOUNDS (VOC) AND HIGH VOLUME LOW PRESSURE (HVLP) PAINTING INFORMATION TO INCREASE
PAINT TRANSFER EFFICIENCY
MODELS:
ALL 1991 AND PRIOR CARS AND TRUCKS
Most solvents used in automotive refinishing paints are considered volatile organic compounds (VOC). Volatile organic compounds react chemically
with sunlight and make a variety of pollutants generally referred to as "Photochemical Smog." VOC regulations seek to reduce the amount of VOC
getting into the air and thereby limit pollution. One way to reduce the amount of volatile organic compounds being emitted into the air is to increase the
transfer efficiency rate of the paint material being transferred between the spray gun and the surface being painted.
Different states throughout the country such as California, New York, New Jersey, etc., are in the process of writing VOC emission regulations. As an
example, the South Coast Air Quality Management District (SCAQMD) in Southern California has a Rule 1151 for automotive refinishing that stipulates
the coating applications must be done with either electrostatic application, high-volume low-pressure (HVLP) spray or "such other coating application
methods that are demonstrated to the executive officer to be capable of achieving at least 65 percent transfer efficiency." HVLP spray is defined as "a
coating sprayed by means of a gun that operates between 0.1 and 10.0 psi air pressure."
What is the difference between the high pressure conventional spray guns now in use and the HVLP spray guns being stipulated in VOC regulations? In
a conventional spray gun high pressure air leaves the air cap and rapidly expands. As this air expands, the paint from the fluid tip is blasted apart sending
small paint droplets in all directions at a high velocity. This explosion of paint causes much of the paint to pass or bounce off the surface being painted.
Transfer efficiency on conventional spray guns is usually between 20 percent and 50 percent efficiency. HVLP spray guns utilize lower spray pressures
upon atomization.
The relatively low air pressure causes a much "softer" explosion. The resultant paint droplets are larger and slower. The large volume of air from the air
caps can now better control the flight of the droplets. The paint spray tends to move in one direction. Bounce back and paint passing by the substrate are
reduced resulting in increased transfer efficiency. Transfer efficiency of HVLP guns is usually between 65 percent to 90 percent. The amount of savings
and transfer efficiency varies with the skill of the painter, gun adjustment, air pressures, size of the object being painted, and viscosity of the paint. There
are three types of HVLP spray equipment available including turbine units. air conversion units, and spray guns with the air conversion units inside the
gun.
The turbine unit is normally a portable or wall-mounted unit that generates a high volume of air at between 3 to 7 psi. Due to friction, the turbine
generates heat. The amount of heat generated and the temperature of the air depend on the size of the turbine and length of hose. A large size diameter
air hose is used between the turbine and the HVLP spray gun.
Air conversion units are normally portable or wall-mounted units that use high pressure air from an air compressor and then convert it to a high volume
of air at a lower pressure set on a regulator. Most air regulators on these units do not exceed 10 psi. A large size diameter air hose is used between the air
conversion unit and the HVLP spray gun. These units do not generate any heat, but can be purchased with a heat source.
HVLP spray guns with the air conversion units inside the gun use high pressure air from an air compressor and then convert it to a high volume of air at a
preset pressure normally dependent on incoming air line pressure.
The spray gun cannot exceed 10 psi regardless of how high the incoming air line pressure may be. There are no large diameter air hoses or heat to
contend with. HVLP equipment that can maintain precise control and maintain close to 10 psi air cap pressure will provide better atomization of
materials, especially low VOC paints.
A GM training course on HVLP Paint Application is currently available (Course # 22001.42). Please refer to the GM monthly training schedule for more
information.
