Gloss defects are among the appearance problems in coated products that most often leave the shop floor uncertain about how to judge them.

Gloss is too low.
Gloss is too high.
Gloss differs locally.
Gloss differs between products despite the same paint being used.
The sample panel and mass-produced product do not match in gloss.
The customer says the finish gives a different impression.

When such problems occur, paint tends to be suspected first.

Is the paint batch different?
Is the amount of matting agent different?
Is the paint manufacturer’s quality inconsistent?

Paint can certainly be the cause.
However, gloss defects do not arise from paint alone.

Gloss depends on how light reflects from the coating film surface.
The film’s surface smoothness, underlying surface condition, film thickness, application viscosity, spray conditions, drying conditions, and measurement conditions therefore have major influences.

To resolve gloss defects correctly, reviewing the underlying surface, film thickness, drying conditions, and operating conditions together with the paint is important.

1. Gloss is an appearance quality perceived through reflected light

Gloss is an appearance quality perceived according to how the coating film surface reflects light.

A smooth coating film surface reflects light in an orderly way and appears glossier.
A surface with fine irregularities scatters light and appears less glossy.

Gloss is therefore more than a color issue.
It is a quality characteristic that reflects the condition of the coating film surface itself.

Is the surface smooth?
Is the underlying surface rough?
Is film thickness uniform?
Does the coating film flow out well during drying?
Is the matting agent uniformly dispersed?

These conditions affect gloss.

Treating gloss defects simply as a difference in paint gloss can therefore lead to a mistaken diagnosis.

2. There are several types of gloss defects

Gloss defects do not describe a single phenomenon.

Typical types include the following.

• Overall gloss is low
• Overall gloss is high
• Gloss differs locally
• Gloss varies in patches
• Gloss differs between the sample panel and mass-produced product
• Appearance differs between flat and curved surfaces
• Underlying sanding marks are visible
• Gloss drops only in filled areas
• Gloss changes after drying
• Gloss is inconsistent between batches

When investigating gloss defects, first define the type of defect.

Is it an overall problem?
Is it localized?
Does it affect only certain products?
Does it occur with particular operators or lines?
Does it appear immediately after drying?
Does it appear after some time?

This clarification changes which causes should be suspected.

3. Underlying surface roughness affects topcoat gloss

The underlying surface is the first factor to check as a cause of gloss defects.

Regardless of how good the topcoat paint is, a rough underlying surface makes it difficult to achieve a smooth film surface.
As a result, light scatters and gloss may appear lower.

The following underlying surface conditions require particular attention.

• Rough substrate surfaces
• Remaining sanding marks
• Rough filler surfaces
• Rough primer surfaces
• Insufficient surfacer
• A rough surface after surface preparation
• Remaining irregularities at welded or machined areas

Underlying surface smoothness is extremely important for high-gloss coating.
Trying to hide underlying irregularities with the topcoat alone can cause excessive thickness, sagging, or an inconsistent finish.

When gloss defects occur, first examine the underlying surface condition as well as the topcoat.

4. Absorption by filler and primer causes gloss drop

When coating filler surfaces or highly absorbent underlying surfaces, paint components may be absorbed into the surface, leaving an uneven finish.

This can cause localized gloss drop.

Examples include the following.

• Gloss is low only in filled areas
• Gloss differs only in repaired areas
• The topcoat sinks because the primer is insufficiently dried
• Gloss is inconsistent on highly absorbent substrates
• Gloss differs between sanded and unsanded areas

Such gloss defects may be caused by underlying absorption or drying condition rather than the paint batch.

Possible measures include smoothing the underlying surface, fully drying the primer, applying a sealer, and achieving the required primer film thickness.

Preparing the underlying surface before topcoating is important for stabilizing topcoat gloss.

5. Film thickness variation appears as gloss differences

Film thickness affects gloss as well as color.

Even with the same paint, gloss may appear different in thick and thin film areas.

In thick areas, the film may flow well and produce a smooth surface that appears glossier.
Thin areas, by contrast, are more affected by the underlying surface and may appear less glossy.

Causes of film thickness variation include the following.

• Differences in application technique between operators
• Variation in spray gun distance
• Variation in gun travel speed
• Variation in overlap width
• Variation in delivery rate
• Difficulty coating due to product shape
• Edges and recessed areas
• Effects of fixtures and hanging arrangements

When gloss appears different locally, film thickness must be checked as well as measured with a gloss meter.

A gloss defect may actually be a film thickness defect.

6. Application viscosity and thinner also affect gloss

Application viscosity has a major effect on coating film finish.

Excessively high viscosity may prevent sufficient paint flow and leave a rough surface.
This may lead to a rough finish, orange peel, and insufficient gloss.

Conversely, excessively low viscosity may cause sagging, insufficient film thickness, pigment settling, uneven color, and gloss variation.

The thinner used is also important.

A thinner unsuitable for the paint may make drying too fast or too slow, or affect resin and pigment stability.
As a result, gloss, color, surface texture, and drying characteristics change.

Items to check are as follows.

• Is the recommended thinner being used?
• Is the thinning ratio appropriate?
• Is application viscosity within the standard range?
• Does viscosity change during work?
• Are temperature-related viscosity changes considered?
• Is an alternative thinner being used?

Thinner is more than a diluent.
It is an important material affecting coating film formation, including gloss.

7. Changes in drying conditions also change gloss

Checking drying conditions is extremely important when investigating gloss defects.

After application, paint dries or cures to form a coating film.
Gloss changes according to how the film surface flows and solidifies during this process.

Unsuitable drying conditions cause the following problems.

• Reduced gloss
• Excessive gloss
• Localized gloss differences
• Color changes
• Undercuring
• Only the surface dries while the interior remains undried
• Overbaking

In baking coating in particular, the drying oven setpoint alone is insufficient.
The actual workpiece temperature must be checked.

Temperature differs between locations inside the oven.
Temperature rise differs with part sheet thickness.
Fixtures absorb heat.
The loading quantity changes hot-air exposure.

Such conditions can cause gloss variation even in the same drying oven.

It is important to check drying oven temperature distribution and workpiece temperature when investigating gloss defects.

8. Flash-off time also affects gloss

Flash-off time is the period for which a coated item is left before entering the drying oven.
Depending on the paint, this period affects the finish.

If flash-off time is too short, drying begins before the film has settled sufficiently, potentially causing a rough finish or gloss defects.
If it is too long, the surface condition may change and affect film flow and drying.

Solvent evaporation rate changes especially in environments with high temperature or humidity.
In hot, humid environments such as Vietnam, the effect of flash-off time may be significant.

Items to check are as follows.

• Are coated items placed in the drying oven immediately?
• Is flash-off time standardized?
• Does the time differ between operators or batches?
• Are temperature and humidity effects considered?
• Are conditions the same for sample panel preparation and mass production?

Flash-off time is also an important process condition for stabilizing gloss.

9. Spray conditions determine the coating film surface

In spray coating, air pressure, application distance, gun travel speed, delivery rate, and pattern width affect the coating film surface.

For example, excessive air pressure may make paint mist too fine or cause it to deposit partly dry, reducing gloss.
Excessive application distance can also cause partly dry deposition and a rough finish.
Conversely, too short an application distance can leave the surface excessively wet, leading to sagging or excessive gloss.

Spray-condition items to check are as follows.

• Air pressure
• Application distance
• Gun travel speed
• Delivery rate
• Overlap width
• Pattern width
• Number of application passes
• Differences between operators

If gloss defects occur with particular operators, variations in spray conditions may be responsible.

It is important to define standard conditions instead of relying only on experienced operators’ intuition.

10. The same gloss can look different on curved and flat surfaces

Part shape also requires attention when judging gloss defects.

Even with the same paint, film thickness, and gloss, flat and curved surfaces may look different.
Curved surfaces can concentrate reflected light and appear glossier than flat surfaces.

Rounded areas, corners, recesses, and deeply set areas also have different coating and reflection conditions.
Simply comparing a flat sample panel with a three-dimensional product may therefore produce different impressions.

The following points must be checked when comparing gloss.

• Are flat surfaces being compared with flat surfaces?
• Is a curved area being compared with a flat sample?
• Is the measurement location appropriate?
• Is the gloss meter measurement angle correct?
• Are visual observation angles consistent?

Failure to understand shape-related appearance differences may lead to incorrect judgments.

11. Organize gloss defects through the 4Ms

Organizing gloss defects through the 4Ms makes it easier to distinguish causes.

Material: materials

Check paints, thinners, pigments, matting agents, primers, and fillers.

• Does the paint suit the specified gloss?
• Is the matting agent well dispersed?
• Is the thinner compatible?
• Is compatibility with the primer and filler good?
• Are there differences between paint batches?

Method: methods

Check film thickness, viscosity, application method, flash-off, and drying conditions.

• Is film thickness uniform?
• Is application viscosity within the standard range?
• Is flash-off time appropriate?
• Is the primer sufficiently dried?
• Are spray conditions standardized?

Machine: equipment

Check spray guns, drying ovens, gloss meters, and sanding equipment.

• Is spray gun atomization consistent?
• Is the drying oven temperature distribution stable?
• Is workpiece temperature checked?
• Are gloss meter measurement conditions consistent?
• Are there problems with sanding equipment or underlying surface preparation?

Man: people

Check variations between operators, sanding personnel, and inspectors.

• Does application distance differ between operators?
• Does film thickness variation depend on the operator?
• Does the sanding condition vary?
• Do gloss judgments differ between inspectors?
• Are gloss criteria shared?

12. Management perspective: clarify gloss criteria

Clear gloss criteria are important for preventing gloss defects.

If specifications state only semi-gloss or matte, interpretations may differ between people.

The following items should be defined as necessary.

• Target gloss value
• Tolerances
• Measurement angle
• Measurement location
• Number of measurements
• Conditions for visual judgment
• Limit samples
• Customer-approved samples
• Method for managing measuring instruments

Gloss is a quality characteristic often judged subjectively.
It therefore needs to be controlled through a combination of numerical values, samples, and measurement conditions.

A system is also needed to check film thickness, viscosity, drying conditions, and underlying surface condition from records when gloss defects occur.

13. Particular points to consider in factories in Vietnam

Factories in Vietnam may commonly face the following issues with gloss defects.

• Application viscosity is left to operator judgment
• Film thickness control is insufficient
• Underlying surface sanding criteria are unclear
• Filler and primer drying times are short
• Workpiece temperature in the drying oven is not checked
• Air pressure and application distance differ between operators
• A gloss meter is available, but measurement conditions are inconsistent
• Sample panels and mass-produced products differ in shape and are difficult to compare
• Gloss defects are immediately attributed to the paint batch

Under such conditions, gloss defects may recur even after changing paint.

Reducing gloss defects requires checking the paint, underlying surface, film thickness, drying, operating conditions, and inspection conditions together.

14. Coating the Future with Science and Care

Gloss defects tend to be treated as appearance problems.
In practice, however, they are also signs of variation in process conditions.

Is the underlying surface rough?
Does film thickness vary?
Is viscosity unsuitable?
Are drying conditions unstable?
Do spray conditions differ between operators?
Are inspection conditions unclear?

Distinguishing these factors scientifically helps prevent recurrence.

Clear gloss criteria and operating conditions remove uncertainty for both operators and inspectors.
Unnecessary rework also decreases.
Explanations to customers become clearer.

This is what CoatureVietnam means by bringing science and care to coating.

Science means controlling gloss through the underlying surface, film thickness, drying conditions, and measurement conditions as well as intuition.
Care means using that control to reduce shop-floor uncertainty and workload.

Gloss defects are not solely a paint problem.
A well-organized overall coating process can produce consistent appearance quality.

Practical checklist

When gloss defects occur, check the following.

• Does the underlying surface retain roughness or sanding marks?
• Does gloss drop only in filled or repaired areas?
• Is the primer sufficiently dried?
• Is film thickness within the specified range?
• Are there gloss differences between thick and thin film areas?
• Is application viscosity within the standard range?
• Is the thinner compatible with the paint?
• Is flash-off time standardized?
• Is drying oven temperature distribution checked?
• Is workpiece temperature checked?
• Are air pressure, application distance, and gun travel speed standardized?
• Have gloss meter measurement angle and locations been defined?
• Are curved and flat areas being assessed by the same criteria?
• Are gloss defects being attributed solely to the paint batch?

CoatureVietnam’s perspective

Preventing gloss defects requires integrated control of the underlying surface, film thickness, viscosity, drying conditions, spray conditions, and inspection conditions, as well as the paint.

CoatureVietnam supports stable appearance quality in factories in Vietnam through coating process reviews, diagnosis of film thickness and drying conditions, standardization of spray conditions, and analysis of gloss defect causes.

Gloss is a quality characteristic created by the coating film surface.
To create that surface consistently, it is important to organize the entire coating process scientifically.