When a coating defect occurs, the coating material, operator, application conditions, and drying conditions are often suspected.

These are, of course, important.
However, coating problems quite often originate at an earlier stage.

The cause may be the shape of the product itself.

A shape that is difficult to coat.
A structure that is difficult to hang.
Holes that do not readily drain surface preparation solution.
A box shape whose interior cannot be coated.
A structure that is prone to missed areas after assembly.
A part with nowhere to attach a fixture.

With such products, quality is difficult to stabilize, however much the coating material is improved or the operators try.

Stabilizing coating quality requires considering ease of coating at the product design stage, as well as reviewing the coating process.

CoatureVietnam calls this Design for Coating.

1. Products that are difficult to coat place unreasonable demands on the operation

In coating operations, the difficulty of the work changes greatly with product shape.

Flat, open shapes can be coated relatively evenly.
By contrast, the material has difficulty reaching deep recesses, narrow gaps, enclosed cavities, and complex curved surfaces.

This leads to problems such as the following.

• Missed areas
• Insufficient film thickness
• Uneven film thickness
• Sagging
• Poor drying
• Residual surface preparation solution
• Rust starting inside the product
• Fixture marks
• Longer working time
• Quality differences between operators

The important point is that these are not necessarily operator mistakes.

If the shape is inherently difficult to coat, the operation has to compensate through strained postures, difficult application techniques, and difficult repairs.

Reducing coating defects requires considering shapes that are easy to coat from the design stage, rather than relying solely on shop-floor effort.

2. Product shape has a major influence on coating quality

Product shape directly affects coating quality.

For example, a box-shaped product may be coated neatly on the outside while the material has difficulty reaching the inside and corners.
Small holes and narrow gaps can retain surface preparation solution or moisture.
Solution or moisture can enter gaps at welds or crimped joints and cause rust later.

Corners and edges also tend to have thinner films.
In coatings intended for rust protection, this insufficient thickness can cause rust.

Stabilizing coating quality therefore requires the following perspectives.

• Does the shape allow coating material to reach the surface?
• Does the shape make it easy to achieve the required thickness?
• Does the structure allow surface preparation solution to drain?
• Does the structure dry easily?
• Can fixtures hold the product securely?
• Is the shape easy to inspect?
• Can operators coat it without undue strain?

Coating design must assess not only whether the product can be manufactured, but also whether it can be coated consistently.

3. Structures that retain surface preparation solution cause rust later

Surface preparation before coating may include degreasing, cleaning, and conversion treatment.
It is important for improving film adhesion and rust resistance.

However, some product shapes do not allow preparation solution or moisture to drain readily.

Examples include the following structures.

• An enclosed cavity with only a small hole
• A joint between sheets
• A crimped joint
• A gap at a weld
• A recess that readily collects water
• A pipe shape that is difficult to drain
• A complex internal space

If solution or moisture remains in these areas, rust may develop some time after coating.
Even a neatly coated exterior can conceal corrosion progressing from inside.

For rust-resistant coating, check not only whether the surface is coated, but also whether corrosion-causing factors remain inside or in gaps.

At the product design stage, it is important to provide a structure that allows preparation solution to enter, drain, and dry.

4. Should coating occur before or after assembly?

Whether to coat before or after assembly is another important decision in the coating process.

Coating after assembly may appear to simplify process control because the finished product can be treated as a whole.
However, assembly can create areas that cannot be coated, are difficult for the material to reach, or do not readily drain preparation solution.

Coating individual parts before assembly, by contrast, makes coating easier and helps ensure internal rust protection.
However, more separate parts require measures for fixtures, conveying, inventory control, and preventing scratches during assembly.

The right choice depends on the product.

The following points should guide the decision.

• Will coating after assembly leave missed areas?
• Is internal rust protection required?
• Can preparation solution drain?
• Can the product dry?
• Will assembly damage the coating film?
• Will coating affect dimensions?
• Can productivity and quality be balanced?

The coating process should not be considered separately from assembly.
Design, fabrication, assembly, and coating must be assessed together.

5. A missing hanging hole alone can make a coating line inefficient

On a suspended coating line, workpieces need a point from which they can be hung securely.

However, some products have nowhere to hang them, develop fixture marks on the coated surface when hung, or swing because they are poorly balanced.

Products that are difficult to hang are prone to the following problems.

• Longer working time
• Fixture marks on appearance surfaces
• Film thickness variation caused by product movement
• Missed areas
• A higher risk of falling
• Greater operator workload
• Inability to increase line speed

A hanging hole is more than simply a hole.
It is an important design feature that supports coating quality and productivity.

Provide hanging holes where they will not affect product performance.
Design shapes that are easy to engage with fixtures.
Consider hanging methods that do not obstruct coated surfaces.
Allow hanging in an orientation that drains preparation solution.

Considering these points at the design stage can greatly change coating line stability.

6. Differences in material and sheet thickness also affect drying quality

In bake-curing coating, quality is not determined by the oven setpoint alone.
What matters is the temperature the workpiece actually reaches.

Heat enters materials differently, whether iron, aluminum, stainless steel, resin, or plated products.
Even for the same material, the temperature rise changes with sheet thickness.

Thin parts heat up quickly.
Thick parts are more difficult to heat.
When different materials or thicknesses are mixed, the same drying conditions may produce different states of cure.

This may cause the following problems.

• Undercuring
• Overbaking
• Color differences
• Gloss differences
• Poor adhesion
• Variation in film hardness
• Substrate deformation

For products with strict color and gloss control in particular, temperature differences caused by material and thickness cannot be ignored.

Coating design must check the material and sheet thickness and consider how the workpiece temperature changes inside the oven.

7. Color can also change drying efficiency

Coating color can affect drying conditions as well as appearance.

In drying methods involving heat absorption, such as infrared drying, color can change how readily the coating dries.

In general, black colors tend to absorb heat readily, while white and silver colors tend to reflect it.
The same drying conditions may therefore produce different workpiece temperatures or states of cure for different colors.

This is an important issue for factories with frequent color changes or small-volume production in many colors.

A color change alone may have the following effects.

• Drying time changes
• The state of cure changes
• Gloss changes
• Color differences occur
• Line speed must be adjusted
• Production planning becomes more complex

Coating color is not solely a design issue.
It also affects production conditions.

8. A design that is easy to coat is considerate of operators

Products that are difficult to coat burden operators.

Coating in strained postures.
Inserting the gun into deep recesses.
Hanging heavy parts.
Mounting parts on unstable fixtures.
Checking areas that are difficult to see.
Repeatedly touching up to eliminate missed areas.

These tasks can cause fatigue, safety risks, and psychological strain, as well as quality variation.

A design that is easy to coat is considerate of operators.
A design that is considerate of operators also stabilizes quality.

Operators can coat without undue strain.
Products can be hung securely on fixtures.
Preparation solution drains.
Drying is easy.
Inspection is easy.
Little rework is needed.

Creating these conditions improves productivity and quality in the coating process.

9. Consider Design for Coating through 4M

Organizing Design for Coating through 4M makes it easier to apply in practice.

Material: materials

Check the substrate, surface treatment, preparation chemicals, and compatibility with the coating material.

Check the following points.

• Is the substrate suitable for coating?
• Is there any oil, mold release agent, or chemical residue on the surface?
• Is compatibility with the preparation chemicals satisfactory?
• Does the substrate’s heat tolerance suit the drying conditions?
• Are different materials mixed in the product?

Method: methods

Check the application method, hanging method, assembly sequence, surface preparation, and drying method.

Check the following points.

• Should coating occur before or after assembly?
• Is the hanging orientation appropriate?
• Does the coating sequence minimize missed areas?
• Does the method allow preparation solution to drain?
• Does the orientation allow easy drying?

Machine: equipment

Check fixtures, conveying, surface preparation equipment, the drying oven, and booth capacity.

Check the following points.

• Are fixtures available to hang the product securely?
• Is the size manageable inside the coating booth?
• Can solution enter and drain in the preparation equipment?
• Does the product fit inside the oven?
• Can the required workpiece temperature be achieved?

Man: people

Check operator posture, workload, differences in skills, and ease of inspection.

Check the following points.

• Are operators coating in strained postures?
• Can heavy items be handled safely?
• Does coating difficult areas depend on individual operators?
• Can inspectors readily see the relevant surfaces?
• Can the difficulties be addressed through operator training?

10. Management perspective: design, fabrication, and coating must coordinate

Design for Coating cannot be achieved by the coating department alone.

The design department determines product shape.
The fabrication department makes holes, bends, welds, and crimped joints.
The preparation department performs cleaning and conversion treatment.
The coating department forms the coating film.
Quality assurance performs inspection.
Purchasing selects subcontractors and materials.

If these activities are disconnected, the burden is concentrated on the coating operation.

When a coating defect occurs, consider the following points instead of blaming only the shop floor.

• Is there a problem with the designed shape?
• Is the surface condition after fabrication appropriate?
• Does the structure prevent preparation solution from remaining?
• Were hanging methods and fixtures considered at the design stage?
• Are inspection criteria after coating clear?
• Does the assembly sequence support coating quality?

Stable coating quality requires coordination between departments.
Design for Coating is not only a shop-floor improvement; it is a company-wide approach to process design.

11. Challenges particularly likely to occur in Vietnamese factories

Vietnamese factories may be prone to the following challenges.

• Designs from the Japanese headquarters are manufactured locally without modification
• Product shapes do not suit local equipment
• Hanging fixtures are improvised on the shop floor after design
• Operators struggle to process structures that do not drain preparation solution readily
• Operator skill compensates for areas that are difficult to coat
• Frequent color and product changes make setup complex
• Design intent is not communicated to coating subcontractors
• Defect feedback does not reach the design team

Under these conditions, coating defects are likely to recur.

CoatureVietnam’s support should go beyond proposing coating materials.
It should establish an easy-to-coat process that considers product shape, fixtures, application methods, drying conditions, and operator workload.

12. Coating the Future with Science and Care

Design for Coating is directly linked to CoatureVietnam’s concept of “Coating the Future with Science and Care.”

Science means considering coating quality through shape, film thickness, surface preparation, drying, fixtures, and working conditions rather than impressions.
Care means designing for consistent quality without placing unreasonable demands on operators.

There are limits to supporting difficult-to-coat products solely through shop-floor effort.
To truly stabilize quality, we need shapes that are easy to coat, structures that are easy to hang, designs that dry easily, and criteria that are easy to inspect against.

To reduce coating problems, consider the design stage as well as the coating process.
That is Design for Coating.

Practical checklist

Check the following points about the product design and coating process.

• Is the workpiece shape easy to coat?
• Can the interior be coated?
• Does the structure drain preparation solution and moisture?
• Are there any holes or gaps that readily retain solution?
• Were hanging holes and hanging positions considered at the design stage?
• Will fixture marks appear on appearance surfaces?
• Are any parts better coated before assembly?
• Have you checked drying differences caused by material or sheet thickness?
• Do you consider differences in drying behavior between colors?
• Are operators coating in strained postures?
• Are difficult areas being handled solely through shop-floor effort?
• Can design, fabrication, surface preparation, coating, and quality assurance share information?

CoatureVietnam’s perspective

Stable coating quality requires reviewing the product design itself, as well as coating materials and application conditions.

CoatureVietnam supports coating quality and productivity improvements in Vietnamese factories by improving processes, checking fixtures, conveying and drying conditions, and reviewing workpiece shapes.

Product design that is easy to coat is an important approach not only to stabilizing quality, but also to reducing operator workload and creating a coating process that is considerate of the workplace.