Compressed air is extremely important in spray coating.

Atomizing paint into fine droplets.
Delivering paint to the workpiece.
Blowing away debris with air.
Operating valves in automated equipment.
Operating air-driven pumps.
Supporting cleaning work.

Compressed air is used in many parts of the coating process in this way.

However, compressed air is sometimes treated as an ordinary factory utility that can be taken for granted.

The compressor is running.
The pressure gauge looks normal.
Air is coming out.
Therefore, there is no problem.

This is sometimes how the situation is judged.

However, what matters for coating quality is not simply whether the compressor is running.
It is whether the required pressure, air flow, and cleanliness are available at the inlet of the spray gun actually being used.

Unstable compressed air greatly disrupts coating quality.

Poor atomization.
Uneven film thickness.
Surface roughness.
A gritty finish.
Color variation.
Cratering.
Dirt inclusions.
Paint waste.
Excessive adjustments by operators.

Compressed air system problems may be hidden behind these issues.

1. Compressed Air Is Part of the Coating Conditions

When considering coating conditions, many people think of paint, thinner, film thickness, and drying conditions.

These are, of course, important.
However, compressed air is also an important coating condition in spray coating.

Changes in air pressure change paint atomization.
Insufficient air flow makes paint droplets coarser.
Moisture or oil contamination leads to coating film defects.
Unstable air also prevents operators from maintaining stable spraying conditions.

Compressed air is therefore more than just a power source.
It is an important process condition for turning paint into a coating film.

When investigating coating defects, check compressed air conditions as well as the paint and work method.

2. Compressor Outlet Pressure Alone Is Not Enough to Judge the System

The compressor pressure gauge is sometimes used to check compressed air.

However, sufficient pressure at the compressor outlet does not necessarily mean that sufficient air reaches the point of use in coating operations.

This is because there are many sources of resistance between the compressor and the spray gun.

Piping.
Branches.
Filters.
Regulators.
Hoses.
Couplers.
Fittings.
Air dryers.
Other pneumatic equipment being used simultaneously.

These conditions affect pressure and air flow at the spray gun inlet.

Even with sufficient pressure at the compressor outlet, pressure may drop as soon as the spray gun is triggered.
In this situation, operators feel that the air is weak, atomization is poor, or spraying is difficult.

It is important to check compressed air at the actual point of use, as well as at the compressor.

3. Check Dynamic Pressure at the Spray Gun Inlet

Dynamic pressure at the spray gun inlet is important for coating.

Here, dynamic pressure means the pressure while air is actually flowing.
Check the pressure with the spray gun trigger pulled and air flowing.

Pressure when air is not flowing is static pressure.
Static pressure alone does not reveal actual conditions during coating.

For example, pressure may be 0.5 MPa when no air is flowing, but drop significantly as soon as the gun is triggered.
In this case, the required air flow may not be available during coating.

In spray coating, it is important to check dynamic pressure rather than static pressure alone.

Check pressure drops at the point of use, particularly when several operators coat simultaneously, when air blowing is used at the same time, or when automated and manual spraying operate together.

4. Insufficient Air Flow Leads to Poor Atomization

A spray gun uses compressed air to break paint into fine droplets.
This is called atomization.

With insufficient air flow, paint is not broken into sufficiently fine droplets.
Droplets become coarse and are less likely to deposit evenly on the surface.

As a result, the following defects are more likely to occur.

• Rough finish
• Roughness
• Uneven film thickness
• Sagging
• Uneven color
• Uneven gloss
• A rough finish
• Increased paint consumption

When operators feel that atomization is poor, they increase air pressure, change gun distance, or adjust paint output.

However, if insufficient air supply is the underlying cause, operator adjustments alone cannot stabilize the process.

When poor atomization occurs, check compressed air supply conditions as well as paint viscosity and gun settings.

5. Moisture Contamination Causes Cratering and Poor Adhesion

Moisture can enter compressed air.

When air is compressed, moisture in it condenses and forms condensate.
If this moisture is not removed properly, it can travel through the compressed air line to the spray gun.

Moisture entering the paint or coating film may cause the following defects.

• Cratering
• Dirt inclusions
• Poor adhesion
• Blushing
• Uneven coating film
• Drying defects
• Appearance defects

Particular care is needed in high-humidity environments.
Vietnam has periods of high temperature and humidity, making moisture control in compressed air extremely important.

The following equipment and management measures are needed to control moisture.

• Air dryers
• Condensate drainage
• Air filters
• Moisture separators
• Control of piping slopes
• Periodic inspection
• Condensate drainage records

Moisture in compressed air is difficult to see, but it has a major effect on coating quality.

6. Oil Contamination Also Causes Coating Film Defects

Oil can also enter compressed air.

In particular, when using an oil-lubricated compressor, inadequate management may allow small amounts of oil to enter the air line.

Oil on a surface to be coated makes paint more likely to repel from it.
It can also cause poor adhesion and appearance defects.

Typical problems caused by oil contamination are as follows.

• Cratering
• Craters
• Poor adhesion
• Dirt inclusions
• Coating film defects
• Increased recoating

Check the following items to control oil contamination.

• Compressor condition
• Oil separators
• Air filters
• Activated carbon filters
• Condensate management
• Contamination inside piping
• Scheduled replacement intervals

Air for coating requires greater cleanliness than air for general work.

Stabilizing coating quality requires checking air cleanliness as well as pressure.

7. Small Hoses, Fittings, and Couplers Cause Air Shortages

Hoses and fittings are easily overlooked in compressed air systems.

Even if compressor capacity is sufficient, narrow hoses or fittings along the route prevent the required air flow from reaching the gun.

Pay particular attention to the internal diameter of couplers and fittings.

Even if a hose looks large from the outside, a small coupler bore restricts the airflow at that point.

The following conditions readily cause air shortages.

• Narrow hoses
• Long hoses
• Small internal diameters in fittings
• Old couplers
• Many branches
• Many bends
• Clogged filters
• Insufficient regulator capacity

Coating operations require checking the entire air path to the spray gun, as well as the compressor itself.

8. Air Leaks Affect Both Electricity Costs and Quality

Air leaks in compressed air piping or hoses make the compressor work unnecessarily.

This increases electricity costs.
It also increases the compressor load.

Furthermore, numerous leaks make it difficult to maintain the required pressure at the point of use.

Air leaks commonly occur at the following locations.

• Hose connections
• Couplers
• Fittings
• Around regulators
• Old piping
• Valves
• Pneumatic tool connections
• Temporary piping

Air leaks are a hidden cost.
However, leaving them unattended for a long time wastes substantial electrical power.

They also affect coating quality.

Compressed air is not free.
Electricity is used to produce it.

Addressing air leaks saves energy and stabilizes quality.

9. Watch for Air Shortages During Simultaneous Use

Multiple pneumatic devices operate simultaneously in coating operations.

Manual spray guns.
Automatic guns.
Air blowing.
Air-operated pumps.
Pneumatic mechanisms in fixtures.
Cleaning air.
Pneumatic tools in other processes.

There may be no problem when devices are used individually, but air shortages may occur when they operate together.

Particular attention is needed at the following times.

• Several operators coat simultaneously
• Air blowing and coating take place at the same time
• Automated equipment is operating
• The compressor also supplies other processes
• Production volume has increased
• New equipment has been added

When introducing equipment or increasing production, check air capacity under simultaneous-use conditions.

It is important to check sufficiency at peak air demand, as well as during normal operation.

10. Compressed Air Inspection Also Relates to Quality Assurance

Compressed air inspection is not solely a maintenance personnel task.
It also relates to quality assurance.

For example, suppose cratering increases.
The cause may be moisture or oil in compressed air, as well as the paint or substrate.

Suppose surface roughness increases.
The cause may be reduced air pressure at the point of use, as well as the operator’s spraying technique.

Suppose uneven film thickness increases.
The cause may be unstable air supply, as well as gun distance.

Quality assurance needs to examine coating defects in relation to compressed air conditions.

Check the following items.

• Air pressure on the day defects occurred
• Condensate generation conditions
• Filter replacement history
• Compressor abnormality history
• Pressure measurements at the point of use
• Pressure drops during simultaneous use
• Air line cleanliness

Compressed air records make it easier to distinguish possible causes of defects.

11. Common Compressed Air Challenges in Vietnamese Factories

Vietnamese factories are prone to the following compressed air challenges.

Only compressor outlet pressure is checked.
Dynamic pressure at the spray gun inlet is not measured.
Condensate drainage is insufficient.
Moisture control is inadequate in hot, humid conditions.
Air filter replacement intervals are not defined.
Hose and coupler internal diameters are not checked.
Temporary hoses remain in use for long periods.
Air leaks are left unattended.
Simultaneous use with other processes causes air shortages.
The relationship between compressed air and coating defects is not recorded.

These conditions make it difficult to stabilize coating quality.

Compressed air is an invisible coating condition.
That is why it needs to be checked with measurements.

12. Basic Items to Check in a Compressed Air System

The following items provide a practical framework for checking compressed air in the coating process.

1. Compressor capacity

Check whether there is spare capacity relative to the required air flow, operating pressure, and simultaneous-use conditions.

2. Dynamic pressure at the point of use

Check the pressure at the spray gun inlet while actually spraying.

3. Pipe diameter

Check whether the piping is too narrow for the required air flow.

4. Hoses and fittings

Check hose length and internal diameter, and the resistance of couplers and fittings.

5. Moisture management

Check condensate drainage, air dryers, and filter condition.

6. Oil management

Check oil mist, filters, and compressor condition.

7. Air leaks

Check piping, hoses, and connections for leaks.

8. Simultaneous use

Check whether air shortages occur at peak demand.

9. Inspection records

Record pressure, filter replacement, condensate, and abnormality history.

10. Relationship to quality

Check relationships with cratering, dirt inclusions, surface roughness, and uneven film thickness.

13. The Value of Compressed Air Management According to CoatureVietnam

CoatureVietnam does not regard compressed air as merely a factory utility.

Compressed air is an important process condition that determines coating quality.

Good paint cannot produce a good coating film if the air is unstable.
Even skilled operators cannot maintain stable quality if air is insufficient at the point of use.
Even new equipment produces coating film defects if moisture or oil enters the air supply.

CoatureVietnam checks compressed air from the following perspectives.

• Whether the required pressure is available at the point of use
• Whether the required air flow is available
• Whether moisture and oil have been removed
• Whether hoses or fittings are causing air shortages
• Whether air leaks are wasting electricity
• Whether coating defects are related to air conditions

This puts CoatureVietnam’s valued principle, “Coating the Future with Science and Care”, into practice.

Science means measuring and recording pressure, flow rate, moisture, oil, and piping resistance, then identifying the causes.
Care means creating a workplace where operators can confidently produce stable quality without being troubled by unstable air conditions.

Summary

Compressed air determines coating quality.

A running compressor is different from the required air actually reaching the coating operation.

The compressor outlet is not the only place to check.

Dynamic pressure at the spray gun inlet.
Air flow.
Moisture.
Oil.
Hoses.
Fittings.
Filters.
Condensate.
Air leaks.
Pressure drops during simultaneous use.

These factors need to be checked together.

Compressed air system problems may underlie poor atomization, uneven film thickness, surface roughness, cratering, and dirt inclusions.

CoatureVietnam regards compressed air as an important coating process condition and proposes improvements that stabilize quality, improve the working environment, and save energy.