When a coating defect occurs, certain comments often come first.

“Could the coating material be bad?”
“Could this coating material be unsuitable?”
“Would it be better to switch to another coating material?”

Coating materials can indeed be the cause.
A material unsuitable for the substrate can cause poor adhesion.
A material unsuitable for the service environment can cause premature degradation.
A material unsuitable for the drying conditions can cause incomplete curing or drying defects.

However, coating defects are not caused by coating materials alone.

Coating quality is determined by the combination of coating material, substrate, surface preparation, thinner, thinning ratio, film thickness, coating method, drying conditions, equipment, operators, working environment, inspection criteria and service environment.

In other words, coating defects appear in the coating film as the result of the entire process.

That is why a defect requires reviewing the entire process, rather than suspecting only the coating material.

Coating defects are process signals that appear in the film

There are many types of coating defect.

• Poor adhesion
• Drying defects
• Incomplete curing
• Insufficient film thickness
• Excessive film thickness
• Sagging
• Dirt inclusions
• Contamination
• Uneven finish
• Cratering (fisheyes)
• Pinholes
• Blushing
• Blistering
• Cracking
• Gloss defects
• Color differences
• Rust
• Premature degradation

These ultimately appear as abnormalities in the coating film.

However, their causes do not necessarily lie in the film alone.

For example, poor adhesion may be caused by the coating material.
But it may also be caused by oil on the substrate surface, inadequate surface preparation, insufficient drying conditions, excessive film thickness or a change in substrate grade.

A drying defect may be caused by the drying characteristics of the material.
But it may also result from excessive film thickness, an unsuitable thinner, low temperature, poor ventilation or insufficient drying time.

Coating defects are process signals that appear in the film.
Reading these signals correctly is the first step in root-cause analysis.

First check when the defect occurred

When investigating coating defects, the first thing to check is when they occurred.

When did the defect occur?

• During coating?
• Immediately after coating?
• During drying?
• After drying?
• After assembly?
• After shipment?
• During service?

The causes to suspect differ according to when the defect occurred.

For a defect that occurred during coating, check the material condition, thinning ratio, viscosity, spray conditions, working environment and substrate surface.

For a defect that occurred during drying, check film thickness, thinner, temperature and humidity, ventilation, drying time and flash-off time.

For a defect that occurred after drying or assembly, check cure condition, film thickness, film flexibility, adhesion and handling conditions.

For a defect that occurred during service, check the service environment, weather resistance, water resistance, chemical resistance, abrasion resistance and inspection and maintenance status.

Simply checking when the defect occurred makes it easier to narrow down the likely causes.

Next check where the defect occurred

The next item to check is the location of the defect.

Where on the product does the defect occur?

• Over the entire surface?
• Only in certain areas?
• At corners?
• At edges?
• At welds?
• Around holes?
• In recesses?
• On the reverse side?
• At particular jig contact points?

The location is important information for identifying the cause.

Rust at corners or edges may relate to insufficient film thickness or edge coverage.
Defects at welds may relate to spatter, oxides, insufficient sanding or inadequate cleaning.
Drying defects in recesses may relate to excessive film thickness, inadequate ventilation or residual solvent.
Scratches or peeling at jig contact points may relate to handling or fixing methods.

The defect location indicates where to look in the process.

Check whether all products or only some are affected

It is also important to check whether the defect affects all products or only some of them.

If all products are affected, the cause may lie in common conditions such as material selection, surface preparation, drying conditions or process design.

If only some are affected, possible factors include operating variation, a fault in part of the equipment, differences between substrate batches, localized contamination, variation in film thickness or contact during handling.

If only a particular batch is affected, check the substrate batch, condition of the surface preparation bath, coating material batch, any thinner change, production date, temperature and humidity, and equipment conditions.

If defects are frequent with a particular operator or shift, check differences in operating procedures, training, coating speed, thinning decisions and inspection criteria.

Looking at the extent of the defects helps determine whether the cause lies in general conditions or localized conditions.

Check recent changes

A particularly important question when a coating defect occurs is whether anything has changed recently.

Examples of changes to check include the following.

• The substrate changed
• The substrate manufacturer changed
• The substrate batch changed
• Surface preparation conditions changed
• The cleaning agent changed
• The coating material batch changed
• The thinner changed
• The thinning ratio changed
• The coating method changed
• The operator changed
• The spray gun changed
• Drying oven conditions changed
• The production cycle time changed
• The storage location changed
• The packaging method changed
• Inspection criteria changed

Defects often occur after a change.

A change considered minor on the shop floor may have a substantial impact on coating quality.

For example, changing only the substrate manufacturer can change adhesion.
A small change in thinner can change drying characteristics or surface texture.
Simply increasing the oven load can prevent the workpiece from reaching the required actual temperature.

Whenever defects occur, changes must be checked.

Organize causes through 4M

CoatureVietnam emphasizes the 4M perspective when investigating coating defects.

4M refers to Material, Method, Machine and Man.

Rather than simply saying the coating material is bad, breaking a defect down through 4M allows a broader examination of causes.

Material: Examine materials

Material covers factors related to materials.

In coating, materials include not only coating material, but also thinner, hardener, surface preparation chemicals and the substrate.

Examples of points to check include the following.

• Is the coating material suited to the application?
• Has the coating material batch changed?
• Is the specified thinner being used?
• Is an alternative thinner being used?
• Is the hardener correct?
• Is the hardener within its shelf life?
• Are the surface preparation chemicals in normal condition?
• Has the substrate material changed?
• Are there differences between substrate batches?
• Is there oil, rust, mold release agent or dust on the substrate surface?

Material-related defects are not limited to the coating material.

The substrate surface condition or a different thinner can have a major impact on film quality.

Method: Examine methods

Method covers factors related to operating methods and process conditions.

Differences in methods have a major impact on coating quality.

Examples of points to check include the following.

• Is the degreasing method appropriate?
• How long after surface preparation is the product coated?
• Does the thinning ratio follow the standard?
• Is the viscosity measurement method consistent?
• Is the film thickness appropriate?
• Does the number of coats follow the standard?
• Is the flash-off time appropriate?
• Is drying time sufficient?
• Are baking conditions followed?
• Is the actual workpiece temperature checked?
• Are overcoating conditions appropriate?
• Is the inspection method standardized?

Variation in methods leads to variation in quality.

Even with the same coating material, different thinning ratios, film thicknesses or drying conditions produce different results.

Machine: Examine equipment

Machine covers factors related to equipment and instruments.

Equipment condition is directly linked to coating quality.

Examples of points to check include the following.

• Is the spray gun in normal condition?
• Are the nozzle and air cap free from wear or blockage?
• Is pump delivery stable?
• Is there contamination or residue inside the hose?
• Does compressed air contain moisture or oil?
• Is booth air velocity appropriate?
• Are filters being replaced?
• Is the temperature distribution in the drying oven stable?
• Does the actual workpiece temperature reach the required temperature?
• Is the surface preparation equipment in normal condition?
• Are film thickness gauges and measuring instruments calibrated?

If equipment is unstable, quality will not be consistent despite operators’ efforts.

Dirt inclusions, contamination, drying defects, film thickness variation and incomplete curing in particular can be closely related to equipment conditions.

Man: Examine people

Man covers factors related to people.

Coating is a process readily affected by operators’ skills and decisions.

Examples of points to check include the following.

• Do operators understand the standard procedures?
• Are they changing the thinning ratio by intuition?
• Do spray distance or speed vary?
• Are degreased parts being touched with bare hands?
• Are pre-coating checks performed?
• Are defects reported when they occur?
• Are inspectors’ judgment criteria consistent?
• Is there a quality difference between new and experienced operators?
• Are SOPs used on the shop floor?
• Are training and retraining provided?

What matters here is not blaming people.

To reduce variation between people, it is important to advance standardization, training, the use of jigs, visual management and checklists.

Management: Examine management

In addition to 4M, the Management perspective is also important in coating processes.

Weak management prevents recurrence prevention even when the defect cause is known.

Examples of points to check include the following.

• Are customer specifications clear?
• Are coating specifications in place?
• Are SOPs up to date?
• Is change control effective?
• Is there a defect classification table?
• Are inspection records retained?
• Are film thickness data managed?
• Is there a response flow for defects?
• Are recurrence-prevention measures checked?
• Are training records retained?
• Are inspection criteria shared with the customer?

With weak management, the same defects recur.

It is important to change the system to prevent recurrence, rather than simply repairing a defect once.

Points to examine for each type of defect

The points to examine differ by coating defect type.

Poor adhesion

Check the substrate, surface preparation, oil, moisture, mold release agent, wettability, film thickness, and drying and curing conditions.

It is necessary to examine contact between the coating material and substrate surface, rather than only the coating material.

Drying defects

Check film thickness, thinner, thinning ratio, temperature, humidity, ventilation, drying time and flash-off time.

Solvent may remain inside a film even when the surface is dry.

Incomplete curing

Check the mixing ratio, stirring, pot life, baking temperature, baking time and actual workpiece temperature.

For two-component and baking coatings, a dry appearance alone is not enough.

Dirt inclusions and contamination

Check workpiece cleaning, booth environment, air, filters, material filtration, work clothing and handling environment.

It is necessary to examine the entire working environment, rather than only contamination within the coating material.

Sagging

Check viscosity, thinning ratio, film thickness, gun distance, delivery rate, coating speed and workpiece shape.

The issue involves coating methods and film thickness management, not just the coating material.

Color differences and gloss defects

Check stirring, film thickness, drying conditions, number of coats, material batch, measurement light source and evaluation criteria.

Differences in visual appearance also need to be considered.

Rust and premature degradation

Check the substrate, surface preparation, film thickness, edge treatment, primer, service environment and inspection status.

For outdoor or salt-exposed environments, actual service conditions also need to be considered.

Changing the coating material may not solve the problem

Changing the coating material is one option when defects occur.

However, changing the material alone may not resolve the root cause.

For example, poor adhesion caused by inadequate degreasing may recur even after changing the material.
For drying defects caused by excessive film thickness, application conditions need to be reviewed before changing the material.
For incomplete curing caused by insufficient actual workpiece temperature, oven conditions need to be checked rather than the coating material.
For dirt inclusions caused by dust in the booth, environmental improvements are needed more than a material change.

Changing the coating material can sometimes bring temporary improvement.
However, if process-related causes remain, the same or a different defect may recur.

That is why causes need to be organized first when responding to defects.

Move from blame to building better systems

When coating defects occur, discussions on the shop floor readily turn to responsibility.

The coating material is bad.
The operator is at fault.
The equipment is at fault.
Inspection is at fault.
Management is at fault.

However, searching for someone to blame does not prevent recurrence.

What is truly needed is to organize the causes and create a system that prevents recurrence.

If operators easily make mistakes, make the procedure clearer.
If film thickness varies, standardize coating conditions.
If surfaces are recontaminated after preparation, change the storage method.
If inspection criteria are unclear, create acceptance-limit samples.
If the actual workpiece temperature is insufficient, recheck drying conditions.
If defect information is not retained, establish a recording format.

Coating defects are not a reason to blame the shop floor.
They are information for improving the process.

Include a root-cause analysis perspective in MVR

When a sales team member receives a customer’s inquiry about a defect, simply recording what was said is not enough.

The MVR needs to include information that supports root-cause analysis.

For example, include the following items.

• Defect phenomenon
• Time of occurrence
• Location of occurrence
• Frequency of occurrence
• Affected product
• Coating material used
• Thinner used
• Substrate
• Surface preparation
• Thinning ratio
• Film thickness
• Drying conditions
• Working environment
• Recent changes
• Customer’s hypothesis
• CoatureVietnam’s hypothesis
• What to check next

With this information, a sales report becomes an entry point for technical response rather than simply a communication.

CoatureVietnam’s sales team does more than sell products.
It has a role in accurately organizing customers’ shop-floor issues and connecting them to the next action.

Points to check on the shop floor

When coating defects occur, check the following points.

• When did the defect occur?
• Where did the defect occur?
• Are all products, some products or a particular batch affected?
• Have there been recent changes?
• Has the substrate changed?
• Are the surface preparation conditions appropriate?
• Is there oil, moisture, rust, dust or mold release agent?
• Are the correct coating material and thinner being used?
• Are the thinning ratio and viscosity recorded?
• Is the film thickness appropriate?
• Are drying and curing conditions followed?
• Is the actual workpiece temperature checked?
• Are there abnormalities in coating equipment or the booth environment?
• Are there differences between operators?
• Are inspection criteria clear?
• Are defect records retained?
• Are recurrence-prevention measures reflected in the SOP?

How CoatureVietnam can help

CoatureVietnam does not treat coating defects as a problem of coating material alone.

We organize defect phenomena and examine causes from the 4M + Management perspective.

Paint Optimizer
We review thinning ratios, film thickness, drying conditions and application methods in existing coating processes to support defect reduction.

Work standards and process management
We support the development of defect-checking items, SOPs, checklists and recurrence-prevention procedures.

Coat Visioner
We make defect data, film thickness data, inspection records and occurrence trends visible for use in root-cause analysis.

Paint Voyager
We support selection of coating materials suited to the application and substrate and check material-related risks.

Local Optimer
When considering locally available alternative coatings, we check not only color and price, but also defect risks, test conditions and suitability for mass production.

Coat Systemizer
We support improvements across the entire coating process, including equipment, booths, drying ovens, handling and surface preparation.

Conclusion

Do not look only to coating materials for the causes of coating defects.

Coating materials can be the cause.
However, coating defects often arise from a combination of substrate, surface preparation, thinner, thinning ratio, film thickness, drying conditions, equipment, operators, environment and management.

When a defect occurs, what matters is not immediately deciding who or what to blame.

When did it occur?
Where did it occur?
Under which conditions did it occur?
What changed?
Where in 4M is there an inconsistency?
Which system should be corrected to prevent recurrence?

It is important to work through these questions one by one.

Coating the Future with Science and Care.
CoatureVietnam supports coating processes that make recurrence less likely, rather than blaming defects.