The basics of appearance quality through material differences, sheet thickness differences and baking temperature

Introduction

When discoloration occurs in metal coating, the coating material is often the first thing suspected.

“The color is different despite the same color code.”
“Only certain areas changed color after baking.”
“The same coating material looks different in different areas.”
“Could the coating material batch be different?”

Such inquiries are common in coating operations.

The material batch, color matching, stirring, film thickness and drying conditions are, of course, important.
However, discoloration in metal coating is not necessarily caused by the coating material alone.

Differences in substrate material.
Differences in sheet thickness.
Effects of welding or bending.
Residual oil or rust-preventive oil.
Temperature variation during baking.
Differences in product placement or position within the oven.

These factors can change hue and gloss between areas even with the same coating material.

This article organizes the basic perspective for examining metal coating discoloration through substrate, fabrication, surface preparation and baking conditions, rather than the coating material alone.

1. Discoloration is more than a mismatch in coating material color

When hearing “discoloration,” many people think of a color difference in the coating material.

Differences in material batches, inadequate stirring, thinning conditions, film thickness and baking conditions can indeed change color.

However, substrate-related conditions also have a major impact in metal coating.

For example, parts with different sheet thicknesses within the same product absorb heat differently during baking.
Thin areas warm quickly, while thick areas are harder to heat.
This can produce differences in film cure condition and appearance.

The metal surface at welds or bends may also differ from the surrounding area.
If these areas are more affected by heat or retain oil or fabrication residue, this can appear as discoloration or gloss differences.

When discoloration is observed, it is important to check the substrate and process alongside the coating material.

2. Different materials can look different even with the same coating

Metal substrates include various types.

Iron.
Galvanized steel.
Stainless steel.
Aluminum.
Surface-treated steel sheet.
Pickled steel sheet.
Steel sheet with rust-preventive oil.

These differ in surface condition, heating behavior, coating adhesion and compatibility with surface preparation.

Even with the same coating material, differences in underlying color, surface roughness, heat absorption or surface treatment can make the finish look different.

The underlying surface is particularly visible with light colors, pale shades, clear coatings, metallic coatings and glossy finishes.

If discoloration due to material differences is suspected, check the following points.

• Are different metal materials used within the same product?
• Are materials with different surface treatments mixed together?
• Do substrate batches differ in surface color or roughness?
• Is the plating or chemical conversion treatment uniform?
• Does the underlying color affect the film appearance?

Discoloration caused by material differences is difficult to identify by looking only at the coating material.
Checking substrate information is the first step.

3. Different sheet thicknesses change heating during baking

Oven temperature control is important for baking coatings.
However, checking only the oven setpoint is insufficient.

What matters is the temperature actually reached by the product itself.

Even in the same oven, heating behavior changes with the part’s sheet thickness and shape.

Thin sheets heat quickly.
Thick sections are harder to heat.
Hollow or complex-shaped parts may have different temperature rises in different areas.

These temperature differences can affect cure condition, hue and gloss.

For example, the following phenomena may occur.

• Only certain areas look overbaked
• Only thick areas remain insufficiently cured
• Gloss differs between areas
• The color looks yellowish
• Insufficiently dried areas change color later

During baking, it is important to check actual product temperature rather than oven air temperature alone.

For products with different sheet thicknesses, temperature-indicating labels or temperature loggers can be used to check the product’s actual temperature history.

4. Welds and bends are priority areas prone to discoloration

Metal parts undergo operations such as welding, spot welding, bending, pressing, cutting and grinding.

These fabricated areas are not in the same condition as flat areas.

Welds have been affected by heat.
Bends may have surface stresses or subtle changes.
Ground areas may differ from surrounding areas in roughness or residual oil.
Cut surfaces or areas around burrs are prone to insufficient film thickness.

Such areas readily develop differences in adhesion, film thickness, drying and appearance.

When discoloration occurs, first check its location.

• Around welds?
• At bends?
• On cut surfaces?
• On ground areas?
• Around holes?
• At edges?
• Where fabrication oil tends to remain?

If discoloration is concentrated in fabricated areas, differences in surface condition caused by fabrication need to be suspected before the coating material batch.

5. Residual oil causes yellowing, stains and uneven gloss

Metal parts may carry rust-preventive oil, fabrication oil, press oil or cutting oil.
If these are not adequately removed before coating, discoloration or staining may occur.

Coating over residual oil can cause the following defects.

• Yellowing
• Stains
• Cratering (fisheyes)
• Poor adhesion
• Uneven gloss
• Drying defects
• Blistering

Oil-related discoloration may be hard to recognize immediately after coating.
It can become apparent after baking, drying or a period of time.

Particular care is needed when the fabrication oil or rust-preventive oil type changes.

The substrate supplier changed the rust-preventive oil.
The oil used in fabrication changed.
Degreasing conditions stayed the same.
In such cases, even a degreasing process that previously worked may leave oil behind.

When discoloration occurs, compatibility between the oil type and degreasing conditions must be checked alongside the coating material.

6. Baking temperature variation changes color and gloss

For baking coatings, temperature and time determine film performance.
Both excessive and insufficient temperature cause problems.

Excessive temperature can cause the following problems.

• Yellowing
• Heat discoloration
• Loss of gloss
• Change in hue
• Over-curing of the film
• Embrittlement

Insufficient temperature causes the following problems.

• Incomplete curing
• Insufficient water resistance
• Insufficient chemical resistance
• Insufficient adhesion
• Later discoloration
• Blocking

Remember that the oven setpoint and actual product temperature are not the same.

Even with a correct oven setpoint, product size, sheet thickness, placement and oven position change the temperature actually experienced by the product.

Furthermore, uneven oven temperature distribution can cause color or gloss variation within the same batch.

For discoloration in baking coatings, the following checks are important.

• Temperature distribution in the oven
• Actual product temperature
• Heat-up time
• Holding time
• Conveyor speed
• Product placement
• Differences between left and right or upper and lower oven positions
• Sheet thickness differences

When discoloration occurs, thermal history needs to be checked alongside coating color differences.

7. Variation in surface preparation also causes discoloration

In metal coating, surface preparation condition affects appearance.

Surface preparation conditions the surface before coating.
It includes degreasing, sanding, acid pickling, chemical conversion treatment, rinsing and drying.

Variation in surface preparation can cause differences in color or gloss after coating.

For example, the following cases may occur.

• Sanding marks remain
• Surface roughness differs between areas
• Chemical conversion coverage varies
• Inadequate rinsing leaves white residue
• Inadequate drying leaves moisture
• Recontamination occurs after surface preparation

The underlying surface is particularly apparent with thin films or glossy finishes.

When discoloration or uneven gloss occurs, it is important to check the uniformity of surface preparation alongside the coating material.

8. Consider discoloration and color differences separately

On the shop floor, “the color is different,” “it has discolored” and “there is a color difference” are sometimes used interchangeably.
However, root-cause analysis requires them to be considered separately.

A color difference is a difference between the reference color and the actual coating color.
Color matching, material batch, film thickness, application conditions and drying conditions are involved.

Discoloration, on the other hand, is a change in color after coating or an apparent color change between areas.
The substrate, oil, heat, chemicals, surface preparation or service environment may be involved.

For example, if the color differs from the reference immediately after coating, check color matching, film thickness and application conditions.
If only some areas change after baking, check heating behavior and substrate differences.
If color changes after use, check ultraviolet light, heat, chemicals, moisture, oil and the service environment.

The words “the color is different” alone do not reveal the cause.
Check when, where and how it changed.

9. Situations prone to discoloration in Vietnamese factories

Several factors in Vietnamese factories can lead to discoloration problems.

9-1. Oven temperature distribution is inconsistent

Temperature differences between oven locations can cause color or gloss variation.
Long or old ovens in particular may have temperature differences between entrance, center and exit, between left and right, or between top and bottom.

9-2. Products with different sheet thicknesses are baked under the same conditions

Thin and thick parts heat differently when baked under the same conditions.
Even with the same oven settings, the films may reach different cure states.

9-3. Degreasing conditions are unsuitable for the oil

After a change in fabrication or rust-preventive oil, previous degreasing conditions may leave oil behind.
Residual oil leads to yellowing, stains, uneven gloss and poor adhesion.

9-4. Substrate batches or surface treatments have changed

A change in substrate surface condition can change appearance even with the same coating material.
Checks during substrate changes are particularly important for plated, pickled and surface-treated steel sheets.

9-5. Operators change baking or thinning conditions by judgment

Shop-floor changes to drying time, conveyor speed, thinning ratio or film thickness create appearance variation.
Following standard conditions is the basis for preventing discoloration.

10. Checking procedure when discoloration occurs

When discoloration occurs in metal coating, checking in the following order makes causes easier to organize.

10-1. Check when it occurred

• Was the color different immediately after coating?
• Did it change after baking?
• Did it change during storage?
• Did it change after shipment?
• Did it change during service?

The possible causes differ with timing.

10-2. Check the affected location

• The entire surface?
• Only certain areas?
• At welds?
• At bends?
• At areas with different sheet thicknesses?
• Products placed in a particular oven position?
• Areas where oil tends to remain?

Knowing the location indicates where to look among substrate, fabrication, heat and surface preparation factors.

10-3. Check substrate information

• Is the material the same?
• Is sheet thickness the same?
• Is surface treatment the same?
• Is the substrate batch the same?
• Are the rust-preventive and fabrication oils the same?

If the substrate has changed, the results may change even with the same coating material.

10-4. Check surface preparation conditions

• Are degreasing conditions appropriate?
• Is any oil left?
• Is rinsing satisfactory?
• Does chemical conversion treatment vary?
• Is drying inadequate?

Surface preparation is the foundation of appearance and adhesion.

10-5. Check baking conditions

• Temperature distribution in the oven
• Actual product temperature
• Drying time
• Conveyor speed
• Product placement
• Sheet thickness differences
• Position in the oven

Baking condition checks need to examine the heat actually experienced by the product, rather than the setpoint alone.

11. Questions for customer visits

For the factory manager

• Which products most commonly show discoloration?
• Is discoloration found immediately after coating, after baking or after shipment?
• Does discoloration cause rework or recoating?
• Are the color-difference and appearance tolerances agreed with the customer clear?

For production engineering

• Are there differences in material or sheet thickness within the same product?
• Does discoloration occur at welds or bends?
• Is oven temperature distribution actually measured?
• Has actual product temperature ever been measured?
• Does color or gloss differ with oven position?

For quality assurance

• Are discolored areas recorded in photographs?
• Is discoloration linked to substrate batch, sheet thickness, fabricated areas and oven position?
• Are color differences measured?
• Are discoloration and color differences recorded separately?

For purchasing

• Is coatability checked when substrate or surface-treated steel specifications change?
• Are changes in rust-preventive or fabrication oil type shared with the coating process team?
• Does switching to cheaper substrates affect coating appearance?

For coating shop-floor managers

• Is residual oil checked?
• Are baking oven settings checked every time?
• Have product placement or loading quantities changed?
• Are discolored areas compared against the substrate, fabrication and oven position?

12. How CoatureVietnam can help

CoatureVietnam can support investigation of metal coating discoloration across the entire process, rather than through the coating material alone.

For example, we can support the following activities.

• Organizing discoloration locations
• Checking color and gloss differences
• Organizing substrate information
• Checking material and sheet thickness differences
• Checking surface preparation conditions
• Checking residual oil
• Checking baking oven temperature distribution
• Checking actual product temperature
• Preparing cause-analysis materials for customers
• Preparing recurrence-prevention SOPs

Changing the coating material alone may not solve discoloration problems.
It is important to check in sequence where the cause lies among substrate, fabrication, heat and surface preparation.

Conclusion

Discoloration in metal coating is not necessarily caused by the coating material alone.

Even with the same material, differences in substrate or sheet thickness, welding or bending, residual oil and baking temperature variation can change color and gloss.

When discoloration occurs, first check the following points.

• When did discoloration occur?
• Where did discoloration appear?
• Are there material or sheet thickness differences?
• Is it concentrated in fabricated areas?
• Is oil or rust-preventive oil left?
• Is baking oven temperature distribution stable?
• Is actual product temperature appropriate?

Treating discoloration merely as a coating color mismatch may lead to the wrong cause.

CoatureVietnam’s principle, “Coating the Future with Science and Care,” means considering substrate, fabrication, surface preparation, heat and coating material separately, rather than dismissing discoloration through perception or arguments about responsibility.

Science means breaking discoloration down into process conditions.
Care means converting the findings into clear check items and standard conditions that help the shop floor prevent recurrence.

Read metal coating discoloration through the substrate and process, as well as the coating material.
This is a basic requirement for consistent appearance quality.