Gloss on coated products is inconsistent.
The same coating material produces different finishes with different operators.
Some parts develop gloss while others lose it.
Flat areas look good, but gloss differs at edges or recessed areas.
Customers say that the impression of the finish differs between batches.

When such gloss defects occur, coating materials or drying conditions are often suspected.

Coating materials and drying conditions are, of course, important.
However, in spray coating, the operating conditions themselves strongly affect gloss.

Particularly important are spray air pressure and spray distance.

Air pressure is too high.
Spray distance is too great.
Gun travel speed is too fast.
Delivery rate is too low.
Overlap width is inconsistent.

Variation in these conditions changes the film surface even with the same coating material.
This can change gloss, texture, film thickness and color appearance.

Consistent gloss requires scientific control of spray conditions, as well as the coating material.

1. Gloss is determined by the condition of the film surface

Gloss is an appearance quality perceived through the way the film surface reflects light.

A smooth surface reflects light in an orderly way and appears glossy.
A rough surface scatters light irregularly and appears less glossy.

Thus, gloss is not determined only by the coating type.
It changes according to the film surface formed during application.

In spray coating, the material becomes a mist and deposits on the workpiece.
The condition of that mist when it deposits affects film flow, surface smoothness, film thickness and drying condition.

Air pressure, spray distance, delivery rate and gun travel speed therefore have a major impact on gloss.

2. Excessive air pressure can reduce gloss

In spray coating, air pressure atomizes the material.
Atomization means breaking the material into small droplets for spraying.

With appropriate air pressure, the material forms a stable mist and deposits evenly on the workpiece.

However, excessive air pressure can cause the following problems.

• The mist becomes too fine
• Material deposits in a partly dried state
• Transfer efficiency falls
• The surface becomes more prone to roughness
• Orange peel becomes more likely
• Gloss decreases
• Overspray increases
• Coating material loss increases

If the coating material begins to dry before reaching the workpiece, the film surface cannot flow smoothly as readily.
The surface may become rough, scatter light irregularly and appear less glossy.

Higher air pressure is not necessarily better.
It must be managed within an appropriate range for the coating material, gun, spray distance and workpiece shape.

3. Insufficient air pressure also makes the finish inconsistent

Air pressure that is too low is also a problem.

Low air pressure can cause incomplete atomization and coarse droplets.
The result may be a rough film surface or uneven film thickness.

Problems likely to occur with insufficient air pressure include the following.

• Poor atomization
• Poor surface texture
• Uneven film thickness
• Sagging
• Uneven coverage
• Gloss variation
• Poor finish at edges or in recesses

Air pressure should be neither too high nor too low.
What matters is setting and maintaining pressure appropriate to the coating material and application conditions.

Operators sometimes adjust air pressure by intuition.
However, consistent appearance quality requires a defined standard value and allowable range.

4. Excessive spray distance causes partly dried deposition

Spray distance is the distance from the spray gun tip to the workpiece.

When the distance is too great, the mist takes longer to reach the workpiece.
Solvent evaporates during that time, and the material may deposit in a partly dried state.

Partly dried deposition makes it harder for the film surface to flow smoothly.
This can cause the following problems.

• Gloss decreases
• The surface texture becomes rough
• Roughness develops
• Film thickness is insufficient
• Transfer efficiency falls
• Overspray increases
• The color looks pale

For products with demanding appearance requirements in particular, spray distance variation readily appears as variation in gloss or texture.

Operators may not realize that their spray distance is too great.
Complex part shapes or awkward working postures can naturally increase the distance.

Spray distance needs to be managed through both operator training and jigs or product orientation.

5. Too short a spray distance can cause sagging or excessive gloss

Care is also needed when the spray distance is too short.

When the gun is too close, coating material tends to concentrate in particular areas.
This increases film thickness and tends to produce a wet film.

In some cases, the finish may appear highly glossy.
However, the following problems also become more likely.

• Sagging
• Excessive film thickness
• Drying defects
• Gloss variation
• Uneven color
• Irregular surface texture
• Coating material buildup at edges

A short spray distance may initially create an attractive glossy appearance.
However, excessive film thickness can lead to inadequate drying or sagging.

Spraying at close range to create gloss may seem effective in the short term, but may not provide consistent production quality.

6. Changes in gun travel speed change film thickness and gloss

The speed at which the spray gun is moved also affects gloss.

With slow gun travel, more coating material deposits at the same location.
The film becomes thicker and may appear glossier.
However, the risk of sagging and drying defects also increases.

With fast gun travel, less coating material deposits.
The film becomes thinner and is more readily affected by the underlying surface.
Gloss may decrease or the color may look pale.

Different gun travel speeds between operators produce different finishes even with the same coating material.

This is variation in operating conditions, rather than a coating material problem.

It may be difficult to quantify gun travel speed completely, but training, example videos, standard work and film thickness measurements can reduce variation.

7. Inconsistent overlap causes variation in gloss and film thickness

Spray coating rarely covers an entire surface within one spray width; several passes are normally overlapped.

If overlap width is inconsistent, film thickness becomes uneven.
This may appear as differences in color or gloss.

Insufficient overlap tends to leave missed or thin areas.
Excessive overlap tends to create thick areas.

Variation in overlap width causes the following problems.

• Patchy gloss
• Uneven color
• Uneven film thickness
• Sagging
• Drying defects
• Appearance variation

Overlap control is particularly important when coating large flat surfaces.
On curved or complex surfaces, operator movements can readily change the overlap width.

Operator training should treat overlap width as an important control item alongside air pressure and distance.

8. Inconsistent delivery rate makes appearance inconsistent

Delivery rate is the amount of coating material discharged from the spray gun.

Excessive delivery increases film thickness and causes sagging or drying defects.
Insufficient delivery may lead to insufficient film thickness, uneven color or inadequate gloss.

Delivery rate changes with the following factors.

• Gun settings
• Coating material viscosity
• Air pressure
• Nozzle condition
• Hose blockage
• Pump pressure
• Coating material temperature
• Filter blockage

Thus, delivery rate is affected by equipment condition as well as operator control.

When gloss defects occur, the condition of the gun, hose, nozzle and pump also needs to be checked, rather than only the operator’s technique.

9. Film thickness measurement shows the results of spray conditions

Air pressure, spray distance, gun travel speed, delivery rate and overlap width.
These conditions ultimately appear in the film thickness.

Film thickness measurement is therefore useful for checking variation in spray conditions.

When an area with different gloss is found, measure its film thickness.
Is the film thicker where gloss is high?
Is it thinner where gloss is low?
Does thickness distribution differ between operators?
Does product shape cause uneven thickness distribution?

These checks make it easier to identify the cause of gloss defects.

It is important to combine judgments of gloss defects with numerical film thickness data, rather than relying only on perception.

10. Standardize rather than blame differences between operators

Spray coating is readily affected by individual skill.

Operators differ in how they hold and move the gun, distance, speed, overlap width and aiming.
Quality variation is therefore likely.

However, simply blaming these differences on insufficient skill does not resolve the root cause.

Standard conditions need to be clarified and put into a form that can be taught.

The following forms of standardization are useful.

• Standard air pressure
• Standard spray distance
• Standard gun travel speed
• Standard overlap width
• Standard film thickness
• Standard coating sequence
• Examples of acceptable and unacceptable results
• Work videos
• Film thickness checking practice
• Regular skill checks

It is important to put experienced operators’ intuitive knowledge into a form that can be passed on to younger operators.

11. Complex shapes make spray distance difficult to maintain

Maintaining a constant spray distance is relatively easy on a flat panel.
However, actual products are not all flat panels.

They include curved surfaces, corners, holes, recesses, deep areas and three-dimensional shapes.
With such shapes, gun distance and spray angle are readily disrupted.

The following problems can result.

• Insufficient film thickness in deep areas
• Thin films at corners
• Thick films on flat areas
• Gloss differences on curved areas
• Poor surface texture in recesses
• Missed areas
• Sagging

For complex shapes, jigs, hanging orientation and coating sequence are important alongside operator skill.

Trying to compensate for difficult shapes through shop-floor skill alone increases the burden on operators.
Jigs and product orientation should be reviewed where necessary.

12. Organize spray conditions through 4M

Using 4M to organize gloss defects caused by spray conditions makes causes easier to separate.

Material: Materials

Check the coating material, thinner, viscosity, pigments and matting agent.

• Is coating viscosity appropriate?
• Is the thinner compatible?
• Is drying too fast?
• Is the matting agent evenly dispersed?
• Is coating material temperature stable?

Method: Methods

Check air pressure, distance, speed, delivery rate and overlap width.

• Is air pressure within the standard range?
• Is spray distance appropriate?
• Is gun travel speed consistent?
• Is overlap width constant?
• Has the coating sequence been set?
• Is the standard film thickness being reproduced?

Machine: Equipment

Check the spray gun, nozzle, hose, pump and pressure gauge.

• Is gun atomization stable?
• Is the nozzle free from wear or blockage?
• Is the hose free from internal blockage?
• Can the pressure gauge be read correctly?
• Is pump pressure stable?

Man: People

Check operator skills, posture and judgment criteria.

• Does spray distance differ between operators?
• Does gun travel speed vary?
• Is the working posture reasonable?
• Are operators compensating for difficult-to-coat areas through skill alone?
• Are training and skill checks performed?

13. Management perspective: Put spray conditions into an SOP

Putting spray conditions into an SOP is important for consistent gloss.

The SOP should include the following items.

• Coating material used
• Thinner used
• Thinning ratio
• Application viscosity
• Standard air pressure
• Standard spray distance
• Standard gun travel speed
• Standard overlap width
• Standard film thickness
• Coating sequence
• Flash-off time
• Drying conditions
• First-piece check items
• Film thickness measurement locations
• Appearance assessment criteria

An SOP is not intended to constrain the shop floor.
It provides a reference for operators to produce consistent quality without uncertainty.

Standardization is particularly valuable in spray coating because much of the work depends on operators’ intuition.

14. Points requiring particular attention in Vietnamese factories

Vietnamese factories may commonly encounter the following issues with spray conditions.

• Air pressure is left to operator judgment
• Spray distance is not standardized
• Gun travel speed and overlap width differ between people
• Film thickness is not measured adequately
• Gloss defects are attributed to the coating material batch
• Spray gun maintenance is inadequate
• Nozzle wear and blockage are not checked
• Poor jigs or hanging methods make distance difficult to maintain
• Operator training depends on experienced workers
• Drying and spray conditions are considered separately

These conditions make color differences, insufficient film thickness, sagging and drying defects more likely, as well as gloss defects.

Consistent gloss requires moving spray conditions from shop-floor intuition to controllable conditions.

15. Coating the Future with Science and Care

Spray coating is strongly affected by skilled operators’ techniques.
However, making quality depend only on operators’ intuition leaves the shop floor continually unstable.

Quality changes when operators change.
Even the same operator produces a different finish depending on their physical condition or workload.
When a defect occurs, operators are blamed without knowing the cause.
Younger operators are difficult to develop.
Judgments differ from those of inspectors.

These conditions create a substantial burden on the shop floor.

That is why spray conditions need to be organized scientifically.

Air pressure.
Spray distance.
Gun travel speed.
Delivery rate.
Overlap width.
Film thickness.
Drying conditions.

Standardizing these conditions makes it easier for operators to work without uncertainty.
Quality assurance can identify causes more readily.
Training younger operators also becomes easier.

This is what CoatureVietnam means by “Coating the Future with Science and Care.”

Science means managing spray conditions as reproducible conditions rather than intuition.
Care means establishing an environment for consistent quality, rather than imposing ambiguous decisions on operators.

Consistent gloss requires reviewing spray conditions as well as the coating material.

Practical checklist

To prevent gloss defects caused by spray conditions, check the following.

• Has standard air pressure been set?
• Is air pressure stable during work?
• Is spray distance standardized?
• Does spray distance differ between operators?
• Is gun travel speed consistent?
• Is overlap width standardized?
• Is delivery rate appropriate?
• Are the nozzle and gun free from wear and blockage?
• Are the hose and pump in stable condition?
• Are differences between operators checked through film thickness measurement?
• Is film thickness measured in areas with gloss defects?
• Is spray distance being disrupted on complex shapes?
• Do jigs or hanging methods create an awkward coating posture?
• Are spray conditions included in an SOP?
• Are training materials available for younger operators?

CoatureVietnam’s perspective

Spray air pressure and spray distance strongly affect gloss, film thickness, texture and color differences.

CoatureVietnam supports consistent appearance quality in Vietnamese factories through observation of coating work, film thickness measurement, spray condition standardization, operator training and reviews of jigs and hanging methods.

Gloss defects are not just a coating material problem.
Organizing operating conditions scientifically enables more consistent coating quality.