When coating problems occur, the following conversations are common on the shop floor.

We should have coated it according to the drawing.
We are using the same paint as always.
The customer says the color is different.
Inspectors disagree on acceptance and rejection.
We measure film thickness, but the criteria are unclear.

Behind these problems, in addition to paint and operator issues, there may be ambiguity in the coating specification.

A coating specification is more than a document.
It is a common language that allows design, production engineering, quality assurance, purchasing, the coating shop floor, subcontractors, and customers to judge by the same criteria.

An unclear specification leaves the shop floor uncertain.
Inspection judgments vary.
Agreements with customers become unstable.
This leads to touch-ups, recoating, delivery delays, and higher costs.

To stabilize coating quality, specifications need to define film thickness, color, gloss, coating coverage, drying conditions, and inspection methods, as well as the paint name.

1. A coating specification exists to remove uncertainty, not simply to restrict the shop floor

Specifications are sometimes seen as documents for strict shop-floor control.
However, their intended purpose is broader.

A good specification leaves the shop floor without uncertainty.

Which paint is used?
Which areas are coated?
What film thickness is needed?
What color is the reference?
What gloss is acceptable?
What drying conditions are used?
What inspection method is used?

Clear answers make decisions easier for both operators and inspectors.

An unclear specification, conversely, leaves the shop floor dependent on experience and intuition.
Experienced staff may cope, but quality varies when personnel change.
Misunderstandings are also more likely when work is subcontracted.

A coating specification is both a control document for stable quality and a document that reduces the shop floor’s decision-making burden.

2. Basic items to include in the specification

At minimum, a coating specification needs to define the following items.

• Workpiece material
• Paint to be used
• Coating process
• Required film thickness
• Coating color
• Gloss
• Areas to be coated
• Areas to remain uncoated
• Surface preparation conditions
• Dilution conditions
• Drying and baking conditions
• Inspection methods
• Permissible ranges
• Repair methods
• Health and safety precautions

These items cannot be treated independently.

For example, film thickness affects rust resistance and appearance.
Color is affected by paint lot, film thickness, drying conditions, and light source.
Gloss is affected by substrate condition, film thickness, drying conditions, and spray settings.
Drying conditions affect substrate heat resistance as well as coating film performance.

A coating specification needs to organize the items as conditions that can be implemented consistently on the shop floor, rather than simply listing them separately.

3. Film thickness is a central item in a coating specification

Film thickness is particularly important in a coating specification.

Film thickness is the thickness of the coating film after drying or curing.
It is generally controlled as dry film thickness.

Film thickness greatly affects coating quality.

Insufficient film thickness causes the following problems.

• Insufficient rust protection
• Show-through of the underlying surface
• Unstable color
• Insufficient durability
• A greater tendency for rust to start at edges

Excessive film thickness also causes problems.

• Sagging
• Drying defects
• Changes in gloss and surface texture
• Increased paint consumption
• Higher costs
• Possible effects on assembly dimensions

A thicker film is therefore not necessarily better.
An appropriate film thickness needs to be defined for the purpose of the coating.

For rust-protection coatings in particular, insufficient film thickness can cause serious problems.
For coatings emphasizing appearance, thickness variation may appear as differences in color or gloss.

Instead of simply stating that film thickness must be controlled, the specification needs to define information such as the following.

• Target film thickness
• Permissible range
• Measurement locations
• Number of measurements
• Measurement method
• Allocation of film thickness between primer and topcoat
• Treatment of edges and areas around holes

Film thickness is fundamental to scientific coating quality control.

4. A color number alone is insufficient to specify color

A coating color may be specified only by its color number or name.

However, a color number alone cannot completely prevent color difference problems in mass production.

Color may appear different depending on the following conditions.

• Paint manufacturer
• Paint lot
• Pigment type
• Film thickness
• Underlying color
• Drying conditions
• Gloss
• Lighting
• Viewing angle
• Sample panel condition

For color specification, it is therefore important to use an actual coated sample panel made with the paint that will be used as the reference.

The specification should include the following.

• Color name or number
• Availability of a reference sample panel
• Availability of limit samples
• Permissible color difference range
• Measurement conditions when using color measurement
• Lighting conditions for visual judgment
• Customer approval history
• Sample panel storage and update rules

Japanese manufacturers may have particularly strict color difference requirements.
However, not every component necessarily requires highly precise color control.

Is it a visible part viewed at close range?
An equipment part viewed from a distance?
An internal component?
A brand color?

Color control levels should be differentiated according to the application.

5. Define gloss in the specification too

Gloss, or sheen, also greatly affects coating quality.

Even with the same color, full gloss, semi-gloss, and matte finishes create very different impressions.
Gloss also indicates variation in substrate condition and application conditions, beyond appearance alone.

Gloss is determined mainly by how light reflects from the film surface.

A smooth surface appears glossier.
A rough surface scatters light and appears less glossy.

Gloss is affected by the following conditions.

• Paint formulation
• Dispersion of matting agents
• Substrate roughness
• Film thickness
• Application viscosity
• Spray settings
• Setting time
• Drying and baking conditions
• Measurement angle
• Workpiece shape

The specification should also define the following gloss information.

• Target gloss
• Permissible range
• Measurement angle
• Measurement locations
• Visual judgment criteria
• Availability of limit samples

Unclear gloss criteria lead to different judgments between inspectors.
Define gloss in the specification to avoid subjective discussions about insufficient or excessive shine.

6. Clearly define coated and uncoated areas

A coating specification needs to define where not to coat, as well as where to coat.

For example, the following areas require care.

• Mating surfaces
• Threaded sections
• Grounding surfaces
• Electrical contacts
• Sliding sections
• Sealing surfaces
• Areas intended for welding
• Areas requiring masking
• Fixture contact areas

Paint on areas that must remain uncoated can cause assembly problems, poor electrical continuity, dimensional defects, and sealing problems.
Conversely, failure to coat required areas leads to inadequate rust protection and appearance defects.

For complex-shaped parts in particular, drawings alone may not communicate coating coverage clearly.
Use photographs, illustrations, samples, and masking instructions in a form that supports shop-floor decisions.

Unclear coating coverage creates uncertainty on the shop floor.
That uncertainty leads to quality variation and rework.

7. Drying and baking conditions affect both film performance and substrate protection

Drying conditions are a very important item in a coating specification.

Insufficient drying prevents the film from achieving its performance.
It may lead to poor adhesion, insufficient curing, inadequate chemical resistance, blocking, discoloration, and gloss defects.

Excessive baking or high temperatures, on the other hand, can adversely affect the substrate.

The substrate’s heat-resistance limit requires particular attention.
If the paint’s baking conditions exceed this limit, the following problems can occur.

• Substrate deformation
• Cracks
• Deterioration of surface treatment layers
• Poor adhesion
• Color changes
• Adverse effects on plastic and rubber components

Oven conditions must not be judged by the set temperature alone.
Actual workpiece temperature is what matters.

Even with the same oven setpoint, workpiece heating varies with material, sheet thickness, shape, fixtures, and loading.
Steel and aluminum, and thick and thin parts, heat differently.

The specification needs to define the following.

• Drying or baking temperature
• Drying time
• Setting time
• How workpiece temperature is defined and assessed
• Substrate heat-resistance conditions
• How oven conditions are verified
• How insufficient drying is checked

Copying drying conditions from paint manufacturer documents is insufficient.
It is necessary to verify that they can be reproduced with the actual shop-floor equipment.

8. Include surface preparation conditions in the specification

Coating quality is strongly affected by surface condition before application.

Oil, release agents, chemicals, rust, dust, or moisture remaining on the substrate surface cause problems after coating.

Typical problems include the following.

• Poor adhesion
• Cissing
• Blistering
• Rust
• Discoloration
• Gloss defects
• Coating film delamination

The coating specification therefore also needs to define surface preparation conditions.

Examples include the following.

• Degreasing method
• Whether conversion treatment is required
• Sanding conditions
• Cleaning conditions
• Drying conditions
• Surface roughness
• Checks for residual moisture
• Checks for residual surface preparation solution

When coating defects occur, examining paint alone may not identify the cause.
Even with the same paint, quality will not stabilize if surface preparation conditions remain unclear.

9. Define inspection methods and permissible ranges

A specification needs to define how to judge the result, as well as what to produce.

Unclear inspection methods cause inconsistent quality assurance judgments.
They also create misunderstandings with customers.

The following coating inspection items should be defined.

• Visual inspection criteria
• Film thickness measurement method
• Color difference measurement method
• Gloss measurement method
• Adhesion test method
• Cure verification method
• Inspection lighting
• Viewing distance
• Areas to be assessed
• Limit samples
• Reinspection rules

Limit samples are particularly useful for visual inspection.
Statements such as minor scratches are acceptable or noticeable dirt nibs are unacceptable still leave room for different judgments.

Providing actual accepted and rejected products as samples and sharing them between the shop floor and quality assurance reduces judgment variation.

10. The specification also matters for purchasing

A coating specification is not only for the shop floor and quality assurance.
It is also important for purchasing.

When purchasing selects paint or a coating subcontractor, unclear specifications encourage comparison by price alone.

However, paints that look similar or have the same color may differ in performance and ease of application.

For example, differences may include the following.

• Rust protection performance
• Hiding power
• Drying characteristics
• Color stability
• Gloss stability
• Ease of application
• Safety
• Repairability
• Lot consistency

Clear specifications allow purchasing to check required performance as well as price.
They also identify what must be maintained when considering alternative paints.

Reducing paint costs is important.
However, substitution without checking required performance may increase defect-related costs later.

11. Specification issues likely at factories in Vietnam

The following specification issues may be common at factories in Vietnam.

• Head-office drawings from Japan specify only a color number
• Permissible film thickness ranges are unclear
• Coating coverage is left to shop-floor judgment
• Drying conditions are copied directly from paint manufacturer documents
• Reproducibility in the actual oven has not been verified
• Visual inspection depends on inspector intuition
• Color sample panels are old
• Customer approval history has not been retained
• Specifications have not been communicated sufficiently to subcontractors
• SDS information has not reached shop-floor operators

Under these conditions, quality is difficult to stabilize no matter how hard the shop floor works.

Organize coating specifications into a form that can be used on the shop floor.
This is an important step toward stable coating quality at factories in Vietnam.

12. Coating the Future with Science and Care

Organizing a coating specification does not mean imposing strict restrictions on the shop floor.

Rather, it enables operators to work without uncertainty.
It allows inspectors to judge fairly.
It enables purchasing to choose materials correctly.
It avoids unnecessary misunderstandings with customers.

Clear specifications reduce the shop floor’s decision-making burden.
They also reduce touch-ups and recoating.
Sharing safety conditions helps protect operators.

This is what CoatureVietnam means by “Coating the Future with Science and Care.”

Science means removing ambiguity from coating conditions and organizing film thickness, color, gloss, drying, and inspection.
Care means using that clarity to reduce uncertainty and burden on the shop floor.

A coating specification should be organized for people as well as for quality.

Practical checklist

Check the following in your current coating specification.

• Is the paint to be used clearly defined?
• Are processes such as primer and topcoat application clear?
• Are target film thickness and permissible range stated?
• Are film thickness measurement locations and methods defined?
• Is there an actual coated sample panel in addition to a color number?
• Has the permissible color difference range been agreed with the customer?
• Are target gloss and permissible range defined?
• Are coated and uncoated areas clear?
• Can drying and baking conditions be reproduced with the shop-floor equipment?
• Has the substrate’s heat-resistance limit been checked?
• Are surface preparation conditions included in the specification?
• Are limit samples available for visual inspection?
• Is the same specification shared with subcontractors?
• Are SDS information and safety conditions shared with the shop floor?

CoatureVietnam’s perspective

Reducing coating problems requires a coating specification organized into a usable shop-floor form.

CoatureVietnam supports stable coating quality at factories in Vietnam through paint selection and the organization of film thickness, color, gloss, drying conditions, inspection criteria, and work standards.

Unclear specifications make coating quality dependent on people.
Clear specifications allow coating quality to be stabilized through a system.