An approach to inspection that supports quality assurance

Inspection is very important in a coating process.

Does the color match?
Is gloss acceptable?
Is film thickness within specification?
Are there particles or unevenness?
Are there runs or scratches?
Is adhesion acceptable?
Is the drying condition appropriate?

These checks help prevent defective products from reaching the customer.

However, an important point needs to be considered.

The purpose of inspection is not only to find defects.

Finding defects through inspection is important.
However, that alone does not stabilize coating quality.

What really matters is feeding inspection findings back into the process and creating a system that prevents the same defects from recurring.

Inspection is therefore a starting point for quality assurance and process improvement, rather than merely a final judgment.

Inspection alone cannot create quality

Inspection is necessary to identify defective products.

However, inspection itself does not add new value to the product.

The coating film is produced by the paint, surface preparation, dilution, application method, film thickness, drying conditions, operators, and equipment.
Inspection checks the result.

Even if inspection identifies a defect, it will occur again if the process cause remains.

For example, sorting out insufficient film thickness at inspection does not address the root cause when it occurs every day.
Removing rejected products during visual inspection does not improve application conditions when uneven color keeps recurring.
If adhesion is poor, strengthening tape testing after completion will not prevent recurrence if surface preparation or drying conditions remain inadequate.

Inspection is necessary.
However, relying on inspection alone will not stabilize quality.

The process creates quality.
Inspection must provide information that improves that process.

From inspection that finds defects to inspection that prevents defects

Traditional inspection checks completed products and removes defective items.

This is necessary to prevent defective products from reaching the customer.

However, this alone keeps the shop floor perpetually busy.
A defect occurs.
Inspection finds it.
The product is touched up.
It is inspected again.
Delivery deadlines become tighter.
The burden on operators increases.

Repeating this cycle exhausts the shop floor.

The real goal should be a process that does not produce defects.

To achieve this, inspection results need to be used as follows.

• Classify the most frequent defects
• Check which process stages produce them
• Identify the products, colors, and lots where they are most frequent
• Check the operating conditions under which they are likely to occur
• Organize causes using the 4M framework
• Revise SOPs
• Incorporate findings into operator training
• Check whether defects recur

Inspection does not end when a defect is found.
It should be used to transform the process so that defects are not produced.

Visual inspection is prone to variation in judgment

Visual inspection is a particularly difficult part of coating inspection.

Visual inspection checks the following defects.

• Uneven color
• Uneven gloss
• Dirt nibs
• Contamination
• Sagging
• Orange peel
• Pinholes
• Scratches
• Cissing
• Dents
• Insufficient hiding power
• Whitening
• Blistering

These defects are often assessed visually.

However, visual inspection judgments readily vary between people.

One person accepts the product.
Another rejects it.
It looks different during the day and at night.
The color looks different under different types of lighting.
The strictness of judgment varies with the inspector’s experience.

Variation of this kind creates confusion on the shop floor.

Quality assurance requires a system that does not rely solely on inspectors’ intuition.

Limit samples and photographic standards are important

Limit samples and photographic standards help reduce variation in visual inspection.

A limit sample is a physical sample showing the boundary between acceptance and rejection.
A photographic standard shows defect types and severity using photographs.

For example, the following criteria can be established.

• Dirt nibs of this severity are acceptable
• Dirt nibs above this size are unacceptable
• A scratch in this location is unacceptable
• A gloss difference of this magnitude is acceptable
• A color difference above this value is unacceptable
• This sag requires repair
• This insufficient film thickness requires recoating

Limit samples and photographic standards make it easier to align judgments among inspectors, operators, managers, and customers.

Appearance quality is particularly difficult to communicate using words alone.

Slight unevenness.
Slightly low gloss.
A noticeable scratch.
An acceptable dirt nib.

People interpret such descriptions differently.

This is why standards should be shared through physical samples and photographs.

Measuring instruments make judgments more objective

Using measuring instruments is also important for visual inspection and quality checks.

Typical measurement items include the following.

• Film thickness
• Color difference
• Gloss
• Viscosity
• Temperature
• Humidity
• Actual workpiece temperature
• Drying time
• Hardness

For example, a film thickness gauge checks whether the coating has reached the required thickness.
A color difference meter quantifies color differences.
A glossmeter quantifies gloss differences.
A temperature and humidity meter records changes in the coating environment.
Oven temperature and actual workpiece temperature measurements verify baking conditions.

Measuring instruments cannot reveal everything.
However, they reduce subjective judgment and make objective checks easier.

Do not assess film thickness inspection by averages alone

Film thickness has a major effect on coating quality.

An excessively thin film may provide insufficient hiding power, rust protection, and durability.
An excessively thick film may cause drying defects, sagging, cracking, and poor adhesion.

Looking only at the average is insufficient for film thickness inspection.

Even if average film thickness is within specification, some areas may be thin and others thick.

The following areas require particular attention.

• Corners
• Edges
• Around holes
• Recesses
• Welds
• Around bolts
• Back surfaces
• Inside complex shapes

Rust may develop from thin areas.
Drying defects or cracking may occur in thick areas.

It is therefore important to standardize measurement locations for film thickness inspection.

Defining measurement locations, the number of measurements, and acceptance criteria makes film thickness inspection more useful for quality assurance.

Color and gloss appearance change with the environment

The inspection environment is also important when checking color and gloss.

The same coated product can look different under a different light source.
The impression of color may differ between indoor lighting and sunlight.
Gloss and metallic appearance may also change with viewing angle.

The following points therefore need to be checked when evaluating color and gloss.

• Which light source is used?
• What viewing angle is used?
• What viewing distance is used?
• What background is used?
• Is a color difference meter used?
• Is a glossmeter used?
• Are limit samples available?
• Are evaluation conditions the same as the customer’s?

A product accepted at the factory may be rejected by the customer.
This can occur because evaluation environments differ.

Consistent evaluation conditions are important for visual inspection.

Link inspection data to process conditions

Collecting inspection data alone does not lead to process improvement.

The key is to link inspection data to process conditions.

For example, use the following approach.

For insufficient film thickness, check dilution ratio, viscosity, spray settings, the operator, and gun distance.
For uneven color, check stirring, the number of application passes, film thickness, drying conditions, and paint lot.
For frequent dirt nibs, check the booth, compressed air, filters, workpiece cleaning, and working environment.
For poor adhesion, check degreasing, surface preparation, substrate surfaces, drying conditions, and film thickness.
For drying defects, check film thickness, thinner, temperature, humidity, ventilation, and drying time.

Inspection results alone do not identify the cause.
Considering them alongside process conditions allows a hypothesis about the cause to be developed.

Create a defect classification table

A defect classification table helps connect coating defects to process improvement.

The table organizes the defects that have occurred by type.

For example, the following categories can be used.

• Color defects
• Gloss defects
• Dirt nibs
• Contamination
• Sagging
• Unevenness
• Cissing
• Pinholes
• Whitening
• Blistering
• Poor adhesion
• Insufficient film thickness
• Excessive film thickness
• Drying defects
• Scratches
• Rust
• Premature deterioration

After classification, record the number of occurrences, location, process stage, lot, and operating conditions.

This reveals defect trends.

Are defects more frequent in a particular color?
With a particular operator?
On particular equipment?
On high-humidity days?
In a particular substrate lot?
Immediately after coating starts?

Such trends make it easier to develop countermeasures.

Do not place responsibility solely on inspectors

Inspectors are sometimes blamed when defective products reach the customer.

Inspectors certainly have an important role.
It is important not to miss defective products.

However, inspectors alone do not create quality.

Coating quality results from materials, surface preparation, application methods, equipment, operators, drying conditions, inspection criteria, and the management system.

No matter how careful inspectors are, the shop floor will not stabilize if the process produces large quantities of defective products.

The priority in quality assurance is not to increase the burden on inspectors.
It is to create a process that does not produce defects.

Inspectors both find defects and provide information for process improvement.

Share inspection criteria with the customer

Sharing standards with the customer is also important for coating quality.

A product accepted within the factory may be rejected by the customer.
Conversely, a factory may inspect too strictly and carry out more touch-ups than necessary.

Quality standards therefore need to be shared with the customer.

Examples of items to share include the following.

• Permissible color difference range
• Permissible gloss range
• Film thickness specification
• Acceptance criteria for appearance defects
• Permissible scratches and dirt nibs
• Inspection distance
• Inspection time
• Inspection light source
• Limit samples
• Whether repairs are permitted
• Recoating criteria

Unclear standards lead to problems after delivery.

Sharing inspection criteria with the customer supports quality assurance and builds trust.

Reflect inspection results in SOPs

When inspection finds a defect, it is important to incorporate that information into SOPs.

For example, if insufficient film thickness is frequent, add spray settings and measurement locations to the SOP.
If dirt nibs are frequent, review procedures for workpiece cleaning, air blowing, booth cleaning, and filter replacement.
If poor adhesion is frequent, standardize degreasing, time control after surface preparation, glove use, and drying conditions.
If uneven color is frequent, review stirring time, the number of application passes, application sequence, and film thickness control.

An SOP is not finished once it has been written.
It should be updated using inspection results and defect data.

With this approach, inspection becomes a system for process improvement rather than simply sorting products.

Organizing inspection and quality assurance through 4M

CoatureVietnam also organizes inspection and quality assurance through 4M.

Material: Materials

• Has the paint lot changed?
• Are the specified thinner and hardener used?
• Has the substrate lot changed?
• Is the condition of surface preparation chemicals stable?
• Are materials stored appropriately?

Method: Methods

• Are inspection methods standardized?
• Are acceptance criteria clear?
• Are film thickness measurement locations defined?
• Are color difference and gloss measurement conditions defined?
• Are there response procedures for defects?
• Is there a system to feed inspection results back into the process?

Machine: Equipment

• Are film thickness gauges, color difference meters, and glossmeters managed appropriately?
• Are measuring instruments calibrated?
• Is inspection lighting appropriate?
• Are there abnormalities in coating equipment, ovens, or booths?
• Is the equipment required for in-process inspection available?

Man: People

• Are inspectors’ acceptance criteria consistent?
• Do operators understand defect criteria?
• Do managers review inspection data?
• Is reporting defects an established practice?
• Are training and retraining provided?

This framework allows inspection to be treated as quality assurance across the entire process, rather than relying solely on individual attentiveness.

Points to check on the shop floor

The following checks are important for connecting inspection to quality assurance.

• Are inspection criteria clear?
• Are limit samples and photographic standards available?
• Do judgments vary between inspectors?
• Are film thickness measurement locations standardized?
• Are color difference, gloss, and film thickness checked numerically?
• Is the inspection environment standardized?
• Is a defect classification table available?
• Are defect counts and locations recorded?
• Are inspection results used for process improvement?
• Are SOPs revised after defects occur?
• Are quality standards shared with the customer?
• Is quality responsibility concentrated solely on inspectors?

How CoatureVietnam can help

CoatureVietnam supports the use of inspection for quality assurance and process improvement, beyond simply finding defects.

Coat Visioner We organize defect data, film thickness data, and inspection results to make quality trends visible.

Work standards and process control We support the development of inspection criteria, limit samples, defect classification tables, SOPs, and operator training materials.

Paint Optimizer Using inspection results, we review dilution ratios, film thickness, drying conditions, and application methods to support stable coating quality.

Local Optimer When evaluating alternative paints, we help compare existing products and candidates using the same inspection criteria.

Coat Systemizer We support process designs that facilitate inspection, fixture designs that facilitate measurement, and equipment introduction based on quality control requirements.

Summary

Inspection does more than find defects.

Inspection is necessary to prevent defective products from reaching the customer.
However, that alone does not stabilize quality.

What really matters is feeding inspection results back into the process and improving the system so that defects are not produced.

Standardize visual inspection using limit samples and photographic standards.
For film thickness inspection, examine variation as well as the average.
Align measurement conditions for color and gloss.
Classify defects and record occurrence trends.
Incorporate inspection results into SOPs and operator training.

Do not let inspection end with a final judgment.
Connect inspection to quality assurance.

This is an important approach to stabilizing coating quality.

Coating the Future with Science and Care
CoatureVietnam supports systems that stabilize quality and allow operators to make clear decisions, rather than turning inspection into a shop-floor burden.