Coating problems are often treated as defects that occurred on the shop floor.

The color is different.
The gloss does not match.
Discoloration has occurred.
The coating film is not uniform.
The product failed the customer’s inspection.

When such problems occur, paint, operators, and application conditions tend to be suspected first. They can certainly be the causes. In practice, however, it is also quite common for the cause of a problem to have been created before application.

Coating quality is not determined only inside the coating booth.
It is designed through the coating purpose, specifications, substrate shape and material, film thickness, drying conditions, color samples, inspection conditions, and working environment.

In other words, the first step in reducing coating problems lies in design before coating, rather than rework after coating.

1. Decide what the coating film must achieve before selecting paint

The purpose of coating is not simply to add color.

Coating films mainly serve the following roles.

• Protect against rust and corrosion
• Improve resistance to scratches and wear
• Increase appearance value through color and gloss
• Provide functions such as heat resistance, chemical resistance, electrical insulation, and sliding properties

What matters here is which purpose takes the highest priority.

The quality criteria to examine differ between parts that prioritize rust prevention and parts that prioritize appearance.
Paint selection and drying conditions also differ between parts requiring heat resistance and parts requiring decorative properties.

In actual workplaces, however, coating specifications are sometimes used without review simply because the paint has always been used, the customer drawing specifies a color code, or the Japanese head office specified it.

In such cases, the basis for judgment becomes unclear when problems occur.

Before selecting paint, the first question to answer is: What do we want this coating film to achieve?

2. Unclear coating specifications create uncertainty for both the shop floor and quality assurance

Coating specifications must clearly define the following items, as well as the paint name.

• Paint to be used
• Process steps such as priming and topcoating
• Required film thickness
• Coating color
• Gloss
• Areas to be coated
• Drying and baking conditions
• Inspection method
• Tolerances

Film thickness is particularly important.

Film thickness directly affects rust prevention, durability, appearance, drying characteristics, and costs. Insufficient thickness may result in inadequate rust prevention or hiding power. Excessive thickness, on the other hand, may lead to sagging, drying defects, and increased costs.

Unclear color and gloss criteria also cause judgments to vary between inspectors.
One person accepts a product while another rejects it.
The customer rejects it, but it looks acceptable to the factory.
Situations like these place a heavy burden on the shop floor.

Coating specifications are more than paperwork.
They provide a common language that allows the shop floor, quality assurance, production engineering, purchasing, and customers to discuss matters using the same criteria.

3. Shapes that are difficult to coat cannot be addressed through shop-floor effort alone

Coating problems can also arise from product shape.

Examples include the following.

• Box shapes whose interiors cannot be fully coated
• Holes and gaps from which surface preparation liquids do not drain readily
• Products that are difficult to hang during coating
• Structures prone to fixture marks on coated surfaces
• Structures that retain process chemicals at welds or crimped joints
• Parts that are difficult to coat after assembly

With such shapes, missed areas, retained liquids, rust, and uneven film thickness are more likely, regardless of how carefully operators apply the coating.

It is important to consider shapes that are easy to coat at the design stage, rather than seeking improvements only from the coating shop floor.

This is the concept of Design for Coating.
It means considering ease of coating, hanging, drying, and inspection from the product design stage.

Product design that facilitates coating does more than stabilize quality.
It reduces operator workload and rework, and improves productivity.

4. Control color differences through conditions, not intuition

Color problems are very common in coating workplaces.

The color code is correct, but the appearance differs.
It was acceptable at the factory but looks different at the customer’s site.
The color shifted when the batch changed.
The sample panel and mass-produced product do not match.

Paint is not the only cause of these problems.

Color appearance changes with the following conditions.

• Paint batch
• Pigment type
• Film thickness
• Drying conditions
• Gloss
• Underlying color
• Lighting
• Viewing distance
• Background color
• The observer’s judgment

A color code alone is therefore insufficient for color control.

A sample panel made with the actual paint must be used as the reference, with limit samples, color-difference tolerances, lighting conditions, and viewing distance defined as necessary.

Discussions of color differences drag on when based only on intuition.
What matters is agreeing in advance on the conditions and limits for accepting a result.

5. Why a product passes at the factory but fails at the customer’s site

Color appearance can change with the light source.

Colors that looked the same under factory fluorescent lighting look different outdoors.
The color looks different in the customer’s inspection room.
When checked at the Japanese head office, it gives a different impression from that seen at the factory in Vietnam.

Such phenomena occur because of differences in pigment composition and light sources.

Relying only on the color code is particularly risky when considering alternative paints.
Colors that look alike may look different under another light source if their pigment compositions differ.

For color matching, it is important to check under conditions close to a standard light source.
Checking in the actual product service environment provides additional assurance.

6. Discoloration is not solely a paint problem

When discoloration occurs, it tends to be attributed first to the paint.
However, its causes can be found throughout the process.

Examples include the following factors.

• Insufficient paint stirring
• Contamination by paint of another color
• Inadequate cleaning of containers and hoses
• Oil on the substrate surface
• Residual surface preparation liquid
• Uneven film thickness
• Unsuitable thinner
• Variation in drying temperature
• Overbaking
• Storage environment
• Effects of sunlight and the atmosphere

Discoloration cannot therefore be explained simply by saying the paint is bad.

Resolving discoloration problems requires reviewing the process in sequence, from paint preparation through application, drying, storage, and service conditions.

Paint stirring and cleaning during color changes are particularly important on the shop floor.
Using paint without sufficient stirring changes pigment concentration and can cause uneven color.
Residual paint of the previous color in hoses, containers, or stirring rods can also cause color contamination.

Preventing color problems requires standardized stirring, cleaning, and verification methods, rather than reliance on experience alone.

7. Consider gloss defects through the underlying surface, film thickness, and drying conditions as well as paint

Gloss defects also tend to be treated as paint batch problems.

However, gloss is not determined by paint alone.
It depends on how light reflects from the coating film surface.

The following conditions therefore have an influence.

• Roughness of the underlying surface
• Absorption by filler or primer
• Sanding marks
• Film thickness variation
• Dispersion of matting agents
• Application viscosity
• Spray air pressure
• Application distance
• Flash-off time
• Baking temperature
• Drying oven temperature distribution

For example, topcoating a rough underlying surface may leave insufficient smoothness and make the gloss appear lower.
Gloss may also appear different between thick and thin film areas.
Excessive air pressure can impair paint mist deposition and reduce gloss.

Preventing gloss defects requires reviewing the underlying surface, film thickness, application conditions, and drying conditions together with the paint.

8. Occupational safety and health are part of coating design

Coating design must consider occupational safety and health as well as quality and costs.

Some solventborne paints and thinners present fire or health risks.
Some pigments and additives in paint also affect human health and the working environment.

When defining coating specifications, the SDS must therefore be reviewed and the following points addressed.

• Is ventilation adequate?
• Is local exhaust required?
• Is the protective equipment appropriate?
• Are ignition sources controlled?
• Have the hazards been communicated to operators?
• Is the disposal method appropriate?

Occupational safety and health should not be checked only at the end.
They must be incorporated into coating design from the beginning.

Operator safety and reduced workload are also important themes in CoatureVietnam’s valued approach of bringing science and care to coating.

9. A 4M plus Management perspective for preventing coating problems

To reduce coating problems, it is important not to assume a single cause.

CoatureVietnam considers the 4Ms and Management effective perspectives for reviewing coating processes.

Material: materials

Paints, thinners, surface preparation chemicals, pigments, fillers, primers, SDSs, and similar items.

Method: methods

Film thickness, viscosity, stirring, application distance, air pressure, drying conditions, inspection methods, and similar factors.

Machine: equipment

Coating booths, spray guns, drying ovens, fixtures, transport systems, exhaust, lighting, measuring instruments, and similar equipment.

Man: people

Operator skills, inspector judgments, training, workload, safety awareness, and similar factors.

Management: control and management

Specifications, standard work, sample panel management, batch control, customer agreements, records, recurrence prevention, and similar activities.

Color differences, discoloration, and gloss defects do not necessarily arise from a single factor.
Organizing issues through the 4Ms plus Management makes it easier to distinguish causes scientifically.

10. Coating the Future with Science and Care

Preventing coating problems means more than simply reducing defects.

Less rework reduces operator workload.
Less recoating reduces material loss and solvent consumption.
Stable quality strengthens customer trust.
Clear specifications reduce uncertainty in shop-floor decisions.

Reviewing coating processes scientifically also means creating systems that care for the people on the shop floor.

Coating the Future with Science and Care
This approach means improving coating as a whole by considering the coating purpose, substrates, processes, equipment, people, and safety, as well as paint.

In Vietnam’s manufacturing workplaces, CoatureVietnam views coating problems as opportunities to improve the entire process, rather than simply as isolated phenomena.
We support coating processes that stabilize customer quality and allow operators to work more safely and with a more positive outlook.

Practical checklist

To prevent coating problems, start by checking the following.

• Is the purpose of this coating clear?
• Have priorities for rust prevention, appearance, and functionality been established?
• Do the coating specifications explicitly state film thickness, color, gloss, and drying conditions?
• Is the substrate shape easy to coat?
• Do hanging holes, fixtures, and assembly sequence suit the coating process?
• Has the color been agreed using an actual coated sample panel?
• Has a color-difference tolerance been defined?
• Are inspection lighting conditions consistent?
• Is the paint stirring method standardized?
• Are there cleaning rules for color changes?
• Are film thickness, viscosity, and drying conditions recorded?
• When gloss defects occur, are the underlying surface, film thickness, and drying conditions checked?
• Is the SDS reviewed and safety information shared with operators?

Related articles

• Decide What the Coating Film Must Achieve Before Selecting Paint
• Essential Items in Coating Specifications: Film Thickness, Color, Gloss, Coating Area, and Drying Conditions
• Design for Coating: Product Design That Facilitates Coating Stabilizes Quality
• Relying on Color Codes Alone Is Risky: Agree on Color Using Actual Coated Sample Panels
• Control Color Differences Through Tolerances, Not Intuition
• Discoloration Is Not Solely a Paint Problem: Consider Substrates, Oil, and Drying Conditions
• Consider Gloss Defects Through the Underlying Surface, Film Thickness, and Drying Conditions as Well as Paint
• Incorporate Occupational Safety and Health into Coating Design: Reviewing SDSs and the Working Environment