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Fundamentals of Corrosion
7.7.1.4 Spray Painting
A more uniform and smoother surface can be achieved with spray painting
than with brushing or rolling because the latter methods tend to leave brush
or stipple marks with irregular thickness. The most common methods of
spray painting are conventional and airless.
The conventional spray method relies on air for paint atomization. Jets
of compressed air, introduced into the stream of paint at the nozzle, break
the stream into tiny droplets that are carried to the surface by the air current. Paint losses from bounce-back or overspray can be high because large
amounts of air are mixed with the paint during application. Such losses have
been estimated to be as much as 30 to 50%.
Some of the disadvantages of conventional air spray applications include:
1. It is slower than airless application.
2. More overspray results than from other methods.
3. It is difficult to coat corners, crevices, etc., because of blowback.
An airless spray system consists of:
1. A plunger pump that supplies high pressure to the paint
2. An airless spraygun
3. A high-pressure-resistant hose
High-viscosity paints are warmed before spraying. This technique has the
following advantages over an air spray system:
1. The sticking rate of paint is increased by 24 to 40%.
2. A thicker film can be applied.
3. The running of paint on the substrate is reduced.
4. Because there is only one hose, it is easier for the operator to use.
5. Higher viscosity paints can be applied.
6. Clean-up is easier.
Because the airless spray process is more efficient than brushing, it is suitable for coating steel structures and bridge girders in the factory. However,
paint loss using the airless spray process is 30 to 40% greater than that of
brushing. The disadvantages of the airless spray system include:
1. Reliance on dangerous high pressure
2. Fan pattern is not adjustable
3. Additional working parts that can cause difficulty
Fundamentals of Corrosion
7.7.1.4 Spray Painting
A more uniform and smoother surface can be achieved with spray painting
than with brushing or rolling because the latter methods tend to leave brush
or stipple marks with irregular thickness. The most common methods of
spray painting are conventional and airless.
The conventional spray method relies on air for paint atomization. Jets
of compressed air, introduced into the stream of paint at the nozzle, break
the stream into tiny droplets that are carried to the surface by the air current. Paint losses from bounce-back or overspray can be high because large
amounts of air are mixed with the paint during application. Such losses have
been estimated to be as much as 30 to 50%.
Some of the disadvantages of conventional air spray applications include:
1. It is slower than airless application.
2. More overspray results than from other methods.
3. It is difficult to coat corners, crevices, etc., because of blowback.
An airless spray system consists of:
1. A plunger pump that supplies high pressure to the paint
2. An airless spraygun
3. A high-pressure-resistant hose
High-viscosity paints are warmed before spraying. This technique has the
following advantages over an air spray system:
1. The sticking rate of paint is increased by 24 to 40%.
2. A thicker film can be applied.
3. The running of paint on the substrate is reduced.
4. Because there is only one hose, it is easier for the operator to use.
5. Higher viscosity paints can be applied.
6. Clean-up is easier.
Because the airless spray process is more efficient than brushing, it is suitable for coating steel structures and bridge girders in the factory. However,
paint loss using the airless spray process is 30 to 40% greater than that of
brushing. The disadvantages of the airless spray system include:
1. Reliance on dangerous high pressure
2. Fan pattern is not adjustable
3. Additional working parts that can cause difficulty