Atomization, distance, and air pressure change film thickness

Air spray coating is a common application method used in many factories.
It uses compressed air to break paint into fine droplets and spray them onto the substrate to form a coating film.

At first glance, air spray coating may appear to be a process in which an operator simply sprays paint.
In practice, however, paint viscosity, thinning ratio, air pressure, delivery rate, gun distance, gun travel speed, overlap width, and drying conditions interact in complex ways.

Even with the same paint, a change in gun distance changes film thickness.
A change in air pressure changes atomization.
Different operator movements create thick and thin film areas.

Air spray coating quality is therefore not determined by paint alone.
It depends on the conditions under which paint is atomized, the distance and speed of application, and how the passes overlap.

1. Atomization is the foundation of air spray coating

In air spray coating, compressed air breaks paint into fine droplets.
This process is called atomization.

Stable atomization helps paint droplets reach the substrate uniformly.
Unstable atomization, by contrast, makes variations in coating film appearance and thickness more likely.

The main conditions affecting atomization are as follows.

• Paint viscosity
• Thinning ratio
• Thinner type
• Air pressure
• Paint delivery rate
• Gun nozzle condition
• Distance between the gun and substrate
• Operator gun handling

Excessively high paint viscosity makes it difficult to break the paint into fine droplets, increasing the likelihood of a rough or grainy finish.
Excessively low viscosity, on the other hand, may lead to sagging or insufficient film thickness.

The first important step in air spray coating is therefore to standardize paint viscosity and thinning conditions.

2. Gun distance changes both film thickness and appearance

The distance between the gun and substrate is extremely important in air spray coating.

If the gun is too close, too much paint tends to deposit in one area.
This may cause sagging, excessive thickness, a rough finish, and uneven gloss.

If the gun is too far away, paint droplets begin drying before reaching the substrate.
This increases the likelihood of roughness, insufficient film thickness, and reduced transfer efficiency.

Operators sometimes work by a subjective sense of slightly farther or slightly closer.
However, these small differences have a major effect on film thickness and finish.

Consistent coating requires defining a standard distance for each product and training operators to maintain the same distance.

3. Higher air pressure is not always better

Increasing air pressure in air spray coating generally makes the paint easier to atomize finely.
This sometimes leads to the belief that higher air pressure produces a better finish.

However, higher air pressure is not always better.

Excessive air pressure makes paint mist more likely to disperse.
This increases paint losses and booth contamination.
Paint may also dry before reaching the substrate, leading to roughness or a rough finish.

Insufficient air pressure, on the other hand, results in inadequate atomization and coarse paint droplets.
This may cause uneven film thickness, a rough finish, and orange peel.

Air pressure must be set to suit paint viscosity, nozzle size, product shape, and required film thickness.

4. The balance between delivery rate and gun travel speed determines film thickness

Coating film thickness is not determined by paint delivery rate alone.

At the same delivery rate, moving the gun slowly deposits a thicker film.
Moving it quickly produces a thinner film.

Film thickness is therefore determined by the following combination.

• Paint delivery rate
• Gun travel speed
• Gun distance
• Overlap width
• Number of application passes
• Paint solids content

Excessive paint delivery makes film thickness easier to achieve, but increases the risk of sagging and drying defects.
Conversely, insufficient delivery leads to inadequate film thickness or hiding.

In air spray coating, it is important to define not only the number of passes, but also the film thickness to be deposited per pass.

5. Inconsistent overlap width causes uneven film thickness

In spray coating, a single gun pattern cannot coat a large surface uniformly.
Successive passes therefore overlap the preceding coated area slightly.

Inconsistent overlap width causes uneven film thickness.

Areas with excessive overlap become thick.
Areas with too little overlap become thin.
This leads to uneven gloss, uneven color, insufficient hiding, and film thickness variation.

Standardizing overlap width is particularly important for large parts and parts with demanding appearance requirements.

Defining the application pattern, work sequence, and application direction stabilizes quality without relying solely on operator skill.

6. Thinner selection also affects the finish

In air spray coating, thinner is more than a means of diluting paint.
It affects drying rate, leveling, the tendency to sag, and surface finish.

A thinner that dries too quickly may cause paint to dry before it has leveled sufficiently on the substrate.
This increases the likelihood of roughness, a rough finish, and insufficient gloss.

Conversely, excessively slow drying increases the risk of sagging, pooling, drying defects, and dirt pickup.

In environments such as Vietnam where temperature and humidity readily fluctuate, it is also important to review thinner drying characteristics according to the season and factory environment.

7. Common problems in air spray coating

The following problems are common issues in air spray coating.

These defects require checking operating and equipment conditions together, rather than judging the paint alone.

8. Basic shop-floor checklist

To stabilize air spray coating, it is important to check at least the following items.

1. Paint viscosity

Viscosity is an important condition indicating how readily paint flows.
If operators make different intuitive thinning adjustments, film thickness and appearance will be inconsistent.

2. Thinning ratio

The amount of thinner to add must be standardized.
Thinning by feel makes quality variation more likely.

3. Air pressure

It directly affects atomization.
Both excessively high and excessively low pressure lead to appearance defects.

4. Delivery rate

It affects film thickness and sagging.
Higher delivery makes film thickness easier to achieve, but also increases the risk of sagging and a rough finish.

5. Gun distance

Both excessive and insufficient distance lead to defects.
It is important to define a standard distance for each product.

6. Gun travel speed

Even at the same delivery rate, changes in gun travel speed change film thickness.
This is a particularly important item in operator training.

7. Overlap width

Inconsistent overlap of spray patterns causes uneven film thickness and uneven color.

8. Drying conditions

Drying time after application, booth conditions, and drying oven conditions also affect the finish.

9. Make conditions visible before blaming operators

Operator skill has a major influence on quality in air spray coating.
However, simply blaming operators when defects occur does not lead to improvement.

The real question is whether conditions are in place that allow operators to produce acceptable products consistently.

For example:

• Has a standard viscosity been defined?
• Has a thinning ratio been defined?
• Is there an air pressure standard?
• Are operators trained in gun distance?
• Have film thickness measurement locations been defined?
• Is there a checking procedure when defects occur?
• Do application methods differ between operators?

If these conditions remain unclear, quality will not stabilize regardless of how hard operators work.

This is exactly what CoatureVietnam means by bringing science and care to coating.
Science means making coating conditions visible through numerical values and calmly checking the causes.
Care means reducing excessive dependence on operator intuition and workload, and moving toward a process in which anyone can consistently produce acceptable products.

10. Directions for improving consistency in air spray coating

The following improvements are effective for stabilizing air spray coating quality.

Create a condition sheet

Define viscosity, thinning ratio, air pressure, delivery rate, gun distance, and drying conditions for each product.

Define film thickness measurement locations

Check areas prone to variation, such as edges, recesses, and overlaps, as well as flat areas.

Standardize operating procedures

Define where to begin application, which direction to follow, and how much overlap to use.

Inspect guns and nozzles

Nozzle wear, clogging, and poor patterns lead to uneven film thickness and surface defects.

Optimize the thinner

Check thinner drying characteristics against temperature, humidity, drying conditions, and required appearance.

Provide training and keep records

Provide training and daily records so operators can reproduce the same conditions.

11. Summary: controlling conditions stabilizes air spray coating

Air spray coating is a highly versatile application method.
It readily accommodates complex shapes and is convenient for high-mix, low-volume workplaces.

However, it is also readily affected by operating conditions.

Film thickness varies.
Sagging occurs.
The surface is rough.
Gloss is inconsistent.
The finish differs between operators.

If these problems occur, first check application conditions rather than suspecting paint alone.

Atomization, distance, air pressure, delivery rate, gun travel speed, and overlap width determine air spray coating quality.
Making these factors visible and standardizing them can reduce operator workload and bring coating quality closer to consistency.

Coating the Future with Science and Care

Improving air spray coating begins with carefully examining shop-floor conditions.

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