A coated product changes color.
The reference panel and production product differ in color.
The coating looks yellowish after drying.
The color fades over time.
Only one area develops uneven color.
The same coating material looks different on different products.

When discoloration problems occur, the coating material is often suspected first.

Could the material lot be faulty?
Could the color mix be wrong?
Could it be a quality problem at the coating manufacturer?

The coating material can, of course, be the cause.
However, discoloration does not arise from the material alone.

Various factors are involved, including oil on the substrate surface, residual preparation solution, film thickness variation, insufficient stirring, unsuitable thinner, drying temperature variation, overbaking, storage conditions, and the service environment.

Correctly resolving discoloration requires checking not only the coating material, but also the entire process before application, during application, after drying, and during use.

1. What is discoloration?

Discoloration is a phenomenon in which the color after coating appears different from the expected or approved color.

There are several types of discoloration.

• The color differs immediately after application
• The color changes after drying
• Yellowing occurs after baking
• Uneven color appears in only part of the product
• The color fades over time
• The color fades in outdoor use
• Chemicals or heat change the color
• The substrate affects the color

The important point is that discoloration has a timeline.

Was it different immediately after application?
Did it change after drying?
Did it change several days later?
Did it change after exposure to the service environment?

The cause cannot be correctly isolated without establishing this timeline.

2. Discoloration causes can lie throughout the process

Discoloration causes cannot be limited to the coating material.

Broadly speaking, causes can lie in the following areas.

• Color specification and reference panels
• Material lots and color mixing
• Stirring and thinning
• Substrates and surface preparation
• Residual oil and chemicals
• Film thickness and application methods
• Drying and baking conditions
• Storage and service environments
• Inspection lighting and judgment conditions

Discoloration therefore does not necessarily have a single cause.

The substrate condition was poor, the film was thin, and drying conditions were also unstable.
Stirring was insufficient and the material lot had also changed.
Preparation solution remained and caused discoloration during baking.
Drying conditions differed between the reference panel and production product.

Several factors can overlap in this way.

Resolving discoloration requires tracing back through the process systematically.

3. First check the color reference and judgment conditions

When discoloration occurs, the first thing to check is not the coating material, but the criteria.

Against what reference is it judged to be discolored?
Is there a reference panel?
Is it current?
Has the customer approved it?
Are the panel and production product viewed under the same conditions?

If this is unclear, the starting point for cause analysis is misplaced.

Check the following items.

• Is there a reference panel?
• Are there limit samples?
• Has the panel deteriorated?
• Are the panel and production product compared under the same light source?
• Is the acceptable color difference range defined?
• Is judgment visual or based on a color difference meter?
• Are gloss differences being mistaken for color differences?

In some cases, the reference panel has aged and changed color rather than the production product.
In others, different inspection lighting merely makes the color appear different.

First establish whether discoloration has actually occurred.

4. Different substrates change color appearance

Even with the same coating material, a different substrate may change color appearance.

Iron, aluminum, stainless steel, plated products, resins, and surface-treated steel sheet differ in substrate condition and heating behavior.
Even on iron, surface roughness, oil, oxidation, and preparation conditions change the resulting finish.

Substrate differences affect discoloration in the following ways.

• The substrate color shows through
• Different heat absorption changes baking behavior
• Heat or solvent affects the surface treatment layer
• Chemicals on the substrate surface affect the film
• Absorption or surface roughness changes gloss
• Coating wettability changes

Thin films and colors with low hiding power are particularly susceptible to substrate influences.

Colors such as white, yellow, and red may be readily affected by the substrate.
For these colors, control of the undercoat, film thickness, and substrate color is important.

5. Residual oil causes discoloration and bleeding

Oil remaining on the substrate before coating causes various problems.

Oil is known to cause poor adhesion and fisheyes.
However, its effects are not limited to those defects.
It can also cause discoloration, bleeding, yellowing, and poor gloss.

Machining oil, rust-preventive oil, fingerprints, grease, machine oil, or mold release agents remaining on the surface may affect the film after application or during drying.

For example, the following phenomena may occur.

• Only part of the product becomes yellowish
• Uneven color appears
• Fisheyes occur
• Gloss changes
• Bleeding appears after drying
• Discoloration appears after some time

In these cases, check degreasing before suspecting the coating material.

In degreasing, it is important to verify that oil has actually been removed, rather than simply that cleaning has been performed.

6. Residual preparation solution and chemicals also cause discoloration

Surface preparation may involve degreasing, acid pickling, conversion treatment, and water rinsing.
These are important for improving film performance.

However, solution or chemicals left on the surface or in gaps can cause discoloration after coating.

Pay particular attention to the following locations.

• Around holes
• Joints
• Welds
• Crimped joints
• Enclosed cavities
• Pipe interiors
• Recesses that do not readily drain water

Coating over residual chemicals or moisture can affect the film during drying or use and cause discoloration, blistering, rust, or poor adhesion.

The purpose of preparation is not simply to perform the treatment.
It is necessary to confirm that the product has been cleaned, drained, and dried afterward.

7. Insufficient stirring is a basic cause of uneven color and discoloration

Coating material does not remain perfectly uniform in the can indefinitely.

Pigments may settle over time.
Ingredients are particularly likely to collect at the bottom in materials containing high-density pigments.

Using the material without sufficient stirring may create different pigment concentrations at the top and bottom and cause color variation.

Insufficient stirring causes problems such as the following.

• Uneven color
• Insufficient hiding power
• Gloss variation
• Differences in the impression of film thickness
• Color differences within a lot
• The color becomes deeper or lighter toward the end of use

Simply instructing people to “mix well” is insufficient.

Stirring time, stirring method, checks for settling, usage time after stirring, and whether filters are used must be standardized.

8. Unsuitable thinner makes color and gloss unstable

Thinner is not merely a diluent.

It affects viscosity, drying, leveling, pigment dispersion, and film formation.
A thinner incompatible with the material may cause uneven color, discoloration, or poor gloss.

Unsuitable thinner may cause the following problems.

• Pigment or resin stability is disrupted
• Agglomeration occurs
• Settling becomes more likely
• Drying is too fast, producing a rough finish
• Drying is too slow, making flow excessive
• Gloss changes
• Color becomes muted
• Film thickness is unstable

Particular care is needed when using alternative thinners.

Changing thinner solely for price or availability can affect film quality.
Check the manufacturer’s recommended thinner, operating temperature, application method, and drying conditions.

9. Film thickness variation appears as color difference

Even with the same material, different film thicknesses can change color appearance.

A thin film may allow the substrate to show through.
A thick film may look deeper in color or have different gloss.

For colors with low hiding power in particular, thickness variation readily appears as a color difference.

Causes of thickness variation include the following.

• Different application techniques between operators
• Variation in application distance
• Variation in gun speed
• Variation in delivery rate
• Difficulty coating the product shape
• Edges and deep recesses
• The influence of fixtures or hanging methods

An apparent color difference may actually be caused by film thickness.

Always check film thickness when investigating discoloration or color differences.

10. Drying conditions are a major factor in discoloration

Drying conditions have a major influence on discoloration.

Temperature and time control are particularly important in bake-curing coating.

Insufficient drying may leave the film undercured and make color or gloss unstable.
Overbaking, conversely, can cause yellowing, fading, gloss changes, or substrate deterioration.

Check the following drying-related items.

• Drying temperature
• Drying time
• Flash-off time
• Temperature distribution in the oven
• Workpiece temperature
• Differences in material and sheet thickness
• Fixture heat capacity
• Product loading arrangement
• Differences in heat absorption between colors

Checking only the oven setpoint is insufficient.
What matters is the temperature actually reached by the workpiece and how long it was held there.

Even in the same oven, heat may enter differently into thick and thin parts, iron and aluminum, or black and white products.

Drying conditions must always be checked in a discoloration investigation.

11. Also check storage and service environments

Discoloration does not necessarily occur only within the coating process.

Storage after coating and the product’s service environment may also cause it.

Examples include the following conditions.

• Direct sunlight
• Ultraviolet light
• High temperature
• High humidity
• Chemicals
• Oil
• Air pollutants
• Contact with packaging materials
• Enclosed storage
• Moisture on the surface

Weather resistance matters for outdoor products.
Heat resistance matters for products used at high temperatures.
Chemical resistance matters for products exposed to chemicals.

Contact with packaging materials or protective films may also leave marks or discoloration.

It is important to determine when discoloration occurred.
Immediately after coating?
During storage?
After transport?
During use by the customer?

This timeline changes the possible causes.

12. Organize discoloration causes through 4M

Organizing discoloration through 4M makes causes easier to isolate.

Material: materials

Check coating materials, pigments, thinners, preparation chemicals, substrates, and oil.

• Are there differences between material lots?
• Is pigment dispersion satisfactory?
• Is the thinner compatible?
• Does oil or chemical residue remain on the substrate?
• Do preparation chemicals remain?
• Are there differences in substrate color?

Method: methods

Check stirring, thinning, thickness, application methods, and drying conditions.

• Is stirring sufficient?
• Is the thinning ratio correct?
• Is thickness within the specified range?
• Are panels and production products coated by the same method?
• Are drying conditions the same?
• Is the undercoat sufficiently dry?

Machine: equipment

Check mixers, coating machines, ovens, lighting, and cleaning equipment.

• Is the mixing equipment appropriate?
• Does the previous color remain in the machine or hoses?
• Is the oven temperature distribution stable?
• Is workpiece temperature checked?
• Is inspection lighting standardized?

Man: people

Check the work and judgments of operators, color-mixing personnel, and inspectors.

• Does film thickness vary between operators?
• Do stirring methods differ between people?
• Do inspectors differ in color judgments?
• Do operators understand discoloration causes?

13. Management perspective: establish SOPs for discoloration prevention

Discoloration measures should not rely on experience alone.

Preventing discoloration requires SOPs for controls such as the following.

• Color reference management
• Material lot records
• Stirring methods
• Thinning ratio control
• Checking the thinner used
• Film thickness control
• Drying condition records
• Workpiece temperature checks
• Cleaning for color changes
• Surface preparation checks
• Inspection lighting conditions
• Cause analysis procedures for complaints

SOPs reduce differences between operators and speed up cause analysis when a problem occurs.

It is important to prevent discoloration through process control, rather than reacting hurriedly after it occurs.

14. Points requiring particular attention in Vietnamese factories

Vietnamese factories need to watch for the following discoloration risks.

• Materials or coated products are stored in hot, humid conditions
• Degreasing is not checked sufficiently
• Shapes do not readily drain preparation solution
• Stirring methods differ between operators
• Alternative thinners are used
• Workpiece temperature in the oven is not measured
• Color-change cleaning is not standardized
• Application conditions differ between panels and production products
• Process condition records are unavailable when a complaint occurs
• Subcontractor drying or stirring conditions are unclear

These can cause color differences, poor gloss, and poor adhesion, as well as discoloration.

For CoatureVietnam, it is important to check the whole process, as well as the coating material.

15. Coating the Future with Science and Care

Discoloration can easily lead to people blaming one another on the shop floor.

The material is bad.
The operator is at fault.
Inspection is too strict.
The customer views it differently.

What is actually needed is scientific isolation of the causes.

Is there a substrate problem?
Does oil remain?
Is stirring insufficient?
Does film thickness vary?
Are drying conditions unstable?
Are reference panels or lighting conditions different?

This approach supports improvements that prevent recurrence, rather than emotional arguments.

This is CoatureVietnam’s concept of “Coating the Future with Science and Care.”

Science means isolating discoloration causes through substrates, processes, equipment, people, and management, rather than treating it solely as a material problem.
Care means creating a reproducible system and reducing the shop-floor burden, instead of blaming operators.

Discoloration is not just a coating material problem.
That is why the entire coating operation must be reviewed and improved.

Practical checklist

Check the following when discoloration occurs.

• Is the reference panel current?
• Are panels and production products compared under the same light source?
• Is the material lot recorded?
• Is the material stirred sufficiently?
• Is the thinning ratio correct?
• Is the thinner compatible with the material?
• Does oil or mold release agent remain on the substrate?
• Does preparation solution or moisture remain?
• Is thickness within the specified range?
• Are panels and production products coated by the same method?
• Do you check workpiece temperature as well as the oven setpoint?
• Do you check thermal differences caused by material or sheet thickness?
• Are containers and hoses cleaned sufficiently at color changes?
• Are there conditions that cause discoloration during storage or transport?
• Do you establish when discoloration occurred?

CoatureVietnam’s perspective

Preventing discoloration requires checking the substrate, surface preparation, stirring, film thickness, drying conditions, and storage environment, as well as coating materials.

CoatureVietnam supports reductions in discoloration problems in Vietnamese factories through defect cause analysis, material and thinner compatibility checks, reviews of drying conditions, and development of shop-floor SOPs.

Discoloration appears as a color problem.
Its causes, however, lie throughout the process.
That is why a scientific view of the whole coating operation matters.