Color may match immediately after coating but change over time

Color matching is important in coating quality.

Does the color match the sample panel?
Does it differ from the customer-approved color?
Are there color differences between batches?
Does it look consistent with adjoining parts?
Do gloss and metallic appearance match?

These are important checks for judging coating quality.

However, color has another important aspect.

It is whether the color remains stable over time.

A finish may look attractive immediately after coating but change color days, weeks, or months later, or after outdoor use.

White becomes yellowish.
Light colors become dull.
Red becomes lighter.
Purple fades.
Outdoor products change color quickly.
Gloss falls, making the color also look different.

These phenomena are yellowing and fading.

Yellowing and fading cannot be judged by initial color alone.
How well color is retained in the coated product’s actual service environment must be assessed.

1. What is yellowing?

Yellowing is a phenomenon in which the coating film becomes yellowish over time.

It is particularly noticeable in white and light-colored coatings.

A film looked white immediately after coating but becomes slightly yellowish with time.
White looks yellowed after baking.
Color changes after storage in a hot environment.
Light colors become dull over time even indoors.

Yellowing is suspected in such cases.

Yellowing is affected by the film’s resin and additives, drying and baking conditions, and service environment, as well as pigments.

Even slight yellowing stands out as an appearance defect in white and light colors.
Color changes over time must therefore be considered as well as initial color.

2. What is fading?

Fading is a phenomenon in which coating film color becomes lighter or less vivid over time.

It is often an issue for outdoor products.

Red becomes lighter.
Purple fades.
Vivid colors become dull.
The color of signs and exterior parts changes.
Only sun-exposed surfaces become lighter.
Indoor-facing and outdoor-facing surfaces look different in color.

Fading is suspected in such cases.

Fading is mainly affected by light, particularly ultraviolet radiation.
Heat, oxygen, moisture, chemicals, and atmospheric pollutants are also involved.

The tendency to fade also changes with pigment type.
More vivid colors may require particular checks of weather resistance.

Fading cannot be identified immediately after coating.
It becomes an issue after time in the service environment.

3. An initial color match does not complete quality assurance

If the coated product’s initial color matches the color sample panel, the color matching initially passes.

However, this alone does not complete quality assurance.

Products used outdoors are affected by strong sunlight, rain, moisture, heat, dust, salt, chemicals, and other factors.
Even indoors, high temperatures, lighting, chemicals, cleaning agents, and oils may have an effect.

Even with an initial match, inability to withstand the service environment leads to color differences over time.

Judging by initial color alone is particularly risky for the following products.

• Outdoor products
• Building materials
• Equipment covers
• Automotive, motorcycle, and agricultural machinery parts
• Electrical equipment exterior parts
• White and light-colored parts
• Parts in vivid colors such as red, yellow, and purple
• Parts used in hot environments
• Parts exposed to cleaning agents or chemicals
• Parts requiring long-term appearance quality

Color matching requires checking how long the color must last as well as whether it matches now.

4. Main causes of yellowing

Yellowing has several causes.

Typical examples are as follows.

• Changes in paint resin over time
• Excessive baking temperature
• Excessive baking time
• Hot storage conditions
• Ultraviolet radiation
• Oxidation
• Effects of chemicals and cleaning agents
• Migration of components from the underlying surface
• Effects of packaging materials and plasticizers
• Deposited dirt and oil

In baking coating, excessive temperature or time may yellow white and light colors.
Underbaking, on the other hand, may leave insufficient film performance and increase susceptibility to later discoloration or contamination.

Color migration from the underlying surface may also make a film look yellowish.
This should be treated as bleeding rather than yellowing.

When judging yellowing, it is important to distinguish changes in the coating film itself, color migration from underneath, and dirt deposition.

5. Main causes of fading

The main cause of fading is light, particularly ultraviolet radiation.

Ultraviolet radiation affects resins and pigments in the film.
Resin deterioration leads to gloss loss and chalking.
Pigment deterioration makes color lighter or changes its hue.

The main factors involved in fading are as follows.

• Ultraviolet radiation
• Heat
• Oxygen
• Moisture
• Rain
• Air pollution
• Salt
• Chemicals
• Pigment weather resistance
• Resin weather resistance
• Insufficient film thickness
• Deterioration of the clearcoat layer
• Surface chalking

Outdoors, fading does not necessarily occur alone.

Gloss loss.
Hazing.
Chalking.
Powdering.
Dirt deposition.
Reduced corrosion protection.

It may progress together with these phenomena.

Outdoor products therefore need checks of gloss retention, chalking, and film deterioration, as well as color difference.

6. Check fading risks for red and purple colors

Color type also matters in fading.

In general, fading in reds, purples, and vivid colors may be noticeable depending on pigment type.

This does not mean all reds and purples are weak.
Highly weather-resistant pigments can enable a design suited to outdoor use.

The important point is not to judge by color name alone.

Red cannot be used.
Purple is weak.
Rather than making such assumptions, check the weather resistance of the pigments and paint specification being used.

Points to check are as follows.

• Is it an outdoor application?
• Will it receive direct sunlight?
• How long must color be retained?
• Has pigment weather resistance been confirmed?
• Are accelerated weathering data or outdoor exposure records available?
• Is clearcoat protection needed?
• Is the balance between initial vividness and weather resistance appropriate?

Vivid colors can be important design elements for customers.
It is therefore important to assess appearance over time as well as the initial appearance.

7. Resin weather resistance is also important

Resins matter in yellowing and fading as well as pigments.

The resin holds the pigments and forms the coating film’s structural framework.
If it deteriorates through ultraviolet radiation, oxygen, heat, or moisture, the film surface condition changes.

Resin deterioration causes the following phenomena.

• Gloss loss
• Hazing
• Chalking
• Powdering
• Surface roughening
• Dirt deposition
• The color looks whitish
• Pigments become more exposed at the surface

What looks like fading may therefore actually be a change in color appearance caused by resin deterioration.

For outdoor applications, paint resin weather resistance must be checked together with pigment weather resistance.

Paint selection must consider the resin system, service environment, and required service life, as well as color.

8. Baking conditions can affect yellowing

In baking coating, baking conditions can affect yellowing.

Baking temperature is too high.
Baking time is too long.
Workpiece temperature rises more than expected.
Uneven oven temperature causes localized overbaking.

Under such conditions, the coating film may yellow.

Baking condition effects are particularly noticeable in white, light-colored, and clear finishes.

Points to check are as follows.

• Is workpiece temperature measured rather than just oven temperature?
• Is the required temperature exceeded excessively?
• Is the required time exceeded excessively?
• Are thick and thin workpieces baked under the same conditions?
• Is oven temperature uneven?
• Has conveyor speed been changed?
• Are only white products yellowing?

When yellowing occurs, baking conditions must be checked as well as paint.

9. Distinguish color migration from the underlying surface and yellowing

When white or light colors become yellowish, yellowing and bleeding may be confused.

Yellowing is a change in which the film itself becomes yellowish.
Bleeding is the migration of color components from the substrate or old film into the topcoat.

They may look similar, but their causes and measures differ.

For yellowing, check paint resin, baking conditions, ultraviolet radiation, heat, oxidation, and similar factors.
For bleeding, check the underlying surface, markings, tar, pitch, wood resin, old film, and whether a sealer is present.

The following checks help distinguish them.

• Is yellowing uniform across the entire surface?
• Is discoloration confined to particular areas?
• Does it coincide with markings?
• Are only repaired areas discolored?
• Are baking conditions excessive?
• Do color components remain on the underlying surface?
• Does the change progress over time?

Rather than judging solely by the resulting color change, examine where the change originates.

10. Prioritize color changes over time in Vietnam’s environment

Vietnam has many environmental conditions that increase yellowing and fading risks.

Strong sunlight.
High temperature and humidity.
The rainy season.
Outdoor storage.
Coastal salt exposure.
Airborne dust and oil.
Chemical mist inside factories.
Hot transport conditions.

These affect coating film color and gloss.

An initial color match is particularly insufficient for outdoor products.
How long the color is retained in the service environment must be checked.

Products requiring attention in Vietnam are as follows.

• Outdoor equipment covers
• Building materials
• Metal exterior products
• Electrical equipment exterior parts
• Signs and display parts
• White and light-colored coated products
• Coated products in vivid reds, yellows, and purples
• Parts used in coastal areas
• Products stored in hot warehouses

Color quality must be considered through stability in the service environment as well as beauty immediately after coating.

11. Shop-floor checkpoints

When yellowing or fading occurs, check the following items.

Defect occurrence

• When did the color change appear?
• Immediately after coating, after baking, after storage, or after use?
• Is it across the whole surface or localized?
• Is it confined to sun-exposed surfaces?
• Is it noticeable in white and light colors?
• Are reds and purples fading?
• Are gloss loss and chalking occurring at the same time?

Service environment

• Will it be used outdoors?
• Will it receive direct sunlight?
• Is the environment hot?
• Is the environment highly humid?
• Is there contact with rainwater?
• Is there coastal salt exposure?
• Is there contact with chemicals or cleaning agents?
• Are hot transport conditions or outdoor storage involved?

Material conditions

• Is the paint suitable for outdoor use?
• Has pigment weather resistance been confirmed?
• Is resin weather resistance sufficient?
• Is the specification resistant to yellowing in white and light colors?
• Has the fading risk of red and purple pigments been checked?
• Has the use of UV absorbers and antioxidants been considered?

Process conditions

• Is baking temperature excessive?
• Is workpiece temperature measured?
• Is baking time excessive?
• Is film thickness appropriate?
• Are drying and curing sufficient?
• Could it be bleeding from the underlying surface?

Evaluation conditions

• Is judgment based only on initial color?
• Are color differences over time checked?
• Is gloss retention checked?
• Are outdoor exposure or accelerated weathering tests being considered?
• Have color-change tolerances been agreed with the customer?

These checks make it easier to narrow down causes of yellowing and fading.

12. Improvement perspectives CoatureVietnam can propose

CoatureVietnam does not view yellowing and fading as simple color mismatches.
We organize them through the 4Ms as issues of color stability over time.

Material: materials

• Is paint resin weather resistance sufficient?
• Are pigment lightfastness and weather resistance sufficient?
• Is the specification resistant to yellowing in white and light colors?
• Are fading risks for reds and purples checked?
• Are clearcoats or additives needed?

Method: methods

• Are changes over time checked as well as initial color?
• Are baking conditions appropriate?
• Is evaluation performed for outdoor applications?
• Have color-difference tolerances been agreed with the customer?
• Is the service environment checked before paint selection?

Machine: equipment

• Is workpiece temperature in the drying oven appropriate?
• Is overbaking occurring?
• Is oven temperature uneven?
• Are facilities available for outdoor exposure evaluation and gloss measurement?
• Are color-difference meters and gloss meters used?

Man: operators

• Can operators distinguish yellowing from bleeding?
• Are they aware that initial color alone is insufficient for judgment?
• Can they check color changes when baking conditions are changed?
• Do they understand fading risks in outdoor products?

For Management, it is important to establish application-specific paint-selection criteria, initial color checks, color-difference checks over time, weather resistance checks for outdoor applications, baking condition control, color sample panel storage rules, and tolerance agreements with customers.

Summary

Yellowing and fading cannot be judged by color immediately after coating alone.

Yellowing is the film becoming yellowish over time.
Fading is color losing intensity due to light, heat, oxygen, moisture, and other influences.

Even when initial color matches the sample panel, color changes develop over time if the film cannot withstand the service environment.

Yellowing is particularly noticeable in white and light colors, while vivid colors such as reds and purples require checks for fading risks.

Measures require comprehensive consideration of pigments, resins, baking conditions, the service environment, ultraviolet radiation, heat, humidity, chemical contact, and outdoor exposure.

Coating the Future with Science and Care
CoatureVietnam goes beyond initial appearance in color matching and supports coating quality customers can use with confidence by considering color stability over time, gloss retention, and the service environment.