A basic perspective for consistent coating quality

When considering coating quality, the first question is often which coating material to use.

Selecting a coating material is, of course, very important.
However, coating quality is not determined by the material alone.

What matters is the kind of coating film that the material actually forms within the coating process.

A coating material is not yet a finished product while it is in the can.
It fulfills its role as a coating film only after it has been applied to a substrate, dried or cured, achieved the required film thickness, adhered firmly to the substrate and delivered its performance in the service environment.

In other words, consistent coating quality requires thinking from the final coating film, rather than just the coating material.

A coating material functions only after it becomes a coating film

Coating materials generally consist of resin, pigments, additives, solvents and other components.

Resin is the principal material that forms the coating film.
Pigments contribute to color, hiding power, corrosion protection and other properties.
Additives are used to adjust coating properties, such as anti-settling, leveling, defoaming, dispersion and weather resistance.
Solvents or water make the coating material suitable for application and assist film formation.

However good these components are, quality will not be consistent unless they form an appropriate coating film in the actual process.

For example, even with the same coating material, a different thinning ratio changes the film thickness.
Different drying conditions change film performance.
Oil or moisture left on the substrate surface causes poor adhesion.
Insufficient film thickness leads to inadequate corrosion protection or durability.
Excessive thickness can also cause incomplete drying or cracking.

The coating material is important.
However, looking only at the material does not reveal the essence of coating quality.

“Dry” and “cured” are not the same

On the shop floor, a coating is sometimes judged dry if its surface feels dry to the touch.

However, depending on the coating type, a dry surface does not necessarily mean that the film has developed its full performance.

There are two broad types of film formation.

One type forms a film as solvents or water leave it.
These are, in effect, coatings that solidify by drying.

The other type forms a film through chemical reactions after application, linking molecules together.
These are coatings that solidify through reaction.

For two-component coatings, baking coatings, powder coatings and similar systems, it is important to control the mixing ratio, pot life, baking temperature, baking time and actual workpiece temperature.

Even when a film looks dry, an incomplete reaction may leave adhesion, chemical resistance, hardness, durability and other properties below the required level.

When considering coating quality, it is therefore necessary to check more than whether a coating is dry.
Check whether it has been properly dried and cured to the point where the required film performance can be achieved.

A hard coating film is not necessarily a good coating film

We sometimes receive requests to make a coating film harder.

Improve scratch resistance.
Improve abrasion resistance.
Make the surface more robust.

Hardness is an important property for these purposes.

However, a harder coating film is not always better.

A hard coating film may resist scratching and abrasion well.
On the other hand, it may be less resistant to bending or impact, making cracking or chipping more likely.

Conversely, a soft coating film may accommodate bending or impact more readily.
However, it may be less resistant to abrasion or scratching.

Film strength therefore cannot be judged by hardness alone.

What type of resistance is actually needed?
Resistance to scratching?
The ability to bend without cracking?
Resistance to chipping on impact?
Resistance to chemicals?
Long service life outdoors?

Without clarifying the purpose, neither appropriate material selection nor appropriate process design is possible.

Film performance needs to be considered from the intended application.

Poor adhesion is not just a coating material problem

Poor adhesion is a particularly important coating problem.

When a coating film peels, it significantly affects not only appearance, but also corrosion protection, durability and quality assurance.

When poor adhesion occurs, the coating material is often suspected.
However, adhesion is not determined by the material alone.

Adhesion is strongly related to the condition of the substrate surface.

If oil, moisture, rust, dust, mold release agent, sanding dust or other contamination remains on the substrate, the film cannot adhere firmly.
Adhesion is also inconsistent if the coating material does not adequately wet the substrate surface.

Blasting and sanding do more than roughen the surface.
They remove contamination and adsorbed moisture and create a fresh surface to which the coating can adhere more readily.
However, waiting too long after treatment can change the surface condition and reduce adhesion.

When investigating poor adhesion, the coating material, substrate, surface preparation, surface condition, time before coating and drying conditions need to be considered together.

Do not manage coating by viscosity alone

Viscosity control is common in coating operations.
Viscosity is an important indicator for managing application workability.

However, coating quality cannot be managed by viscosity alone.

Adding thinner reduces viscosity, but it also changes the proportion of film-forming components in the coating material.

Even if the viscosity makes application easy for the operator, excessive thinning reduces the amount of film-forming material left after drying.
This may lead to insufficient film thickness, hiding power, corrosion protection or durability.

Furthermore, even at the same viscosity, a different thinner can affect drying, leveling, blushing, surface texture and adhesion.

Thinner is not simply a diluting liquid.
It is an important material involved in film formation.

Coating management therefore requires the following items to be considered together.

• Thinning ratio
• Viscosity
• Solids content
• Wet film thickness
• Dry film thickness
• Thinner used
• Drying conditions
• Coating method

It is important to check whether the required film has actually been formed, rather than judging only by ease of application.

Select coating materials from the application, not just the coating name

Coatings have various names and classifications, including acrylic, polyurethane, epoxy, alkyd, powder and waterborne.

These names are useful starting points for understanding a coating.
However, the name alone cannot determine suitability for an application.

Even within the same resin family, performance differs with the manufacturer, formulation, drying conditions, coating method, film thickness and service environment.

The application matters more than the name when selecting a coating material.

• Which substrate will be coated?
• Will it be used indoors or outdoors?
• Is appearance or substrate protection the priority?
• What film thickness is needed?
• Which coating method will be used?
• What drying and curing equipment is available?
• Which test standards must be met?
• Will the product encounter water, oil, chemicals, heat, ultraviolet light or salt in service?

Checking these points improves the accuracy of coating selection.

Selecting a coating material is not just choosing a product name.
It is designing a coating film suited to its application.

Interpret test data in relation to shop-floor conditions

Various test data are available for coating materials and coating films.

Evaluation items include viscosity, density, dispersibility, drying time, adhesion, hardness, impact resistance, water resistance, chemical resistance, weather resistance and corrosion protection.

These test data are important information for quality evaluation.
However, looking only at the numbers does not necessarily allow a correct judgment of shop-floor problems.

What matters is interpreting test data in relation to actual service conditions.

For example, even with good weather-resistance test results, degradation may progress differently in actual service under high temperature and humidity, intense ultraviolet light or salt exposure.

A coating may pass an adhesion test, yet develop poor adhesion in mass production if surface preparation or drying conditions differ.

Test data are a basis for decisions that stabilize shop-floor quality, rather than just documents to submit.

Inspection does more than find defects

Inspection is important for protecting coating quality.

However, the purpose of inspection is not only to find defective products.
Its underlying purpose is to improve the process so that defects are not created.

Appearance judgments can vary between inspectors.
Color, gloss, dirt inclusions, unevenness, scratches and orange peel are particularly prone to subjective assessment.

Acceptance-limit samples, photographic criteria, colorimeters, gloss meters, film thickness gauges and similar tools should therefore be used to clarify judgment criteria as far as possible.

Information on defects found during inspection also needs to be fed back to the process.

At which process did the defect occur?
Under which conditions did it increase?
With which substrates, coating materials, operators or equipment was it common?
Have recurrence-prevention measures been incorporated into the SOP?

Inspection is an entry point for process improvement, rather than simply a final judgment.

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

When a coating defect occurs, it is natural to look for the cause in the coating material.

However, coating defects often have multiple contributing causes.

For example, the following factors may be involved.

• Substrate surface condition
• Surface preparation
• Coating material
• Thinner
• Thinning ratio
• Film thickness
• Coating method
• Drying conditions
• Operator procedures
• Equipment condition
• Temperature and humidity
• Dust
• Storage and handling
• Inspection criteria
• Service environment

Changing the coating material can sometimes bring temporary improvement.
However, if the cause remains in surface preparation, film thickness, drying conditions or the working environment, the same defect may recur.

Defect response requires organizing the causes, rather than looking for someone to blame.
It also requires changing the system so that the defect does not recur.

This also connects to CoatureVietnam’s guiding principle, “Coating the Future with Science and Care.”

Science means breaking down the phenomenon and checking causes from multiple perspectives.
Care means establishing systems that consistently produce good quality, rather than blaming the shop floor.

Coating films have a service life

A coating film should not be evaluated only immediately after application.
It changes over time in its service environment.

Outdoor coating films are affected by ultraviolet light, oxygen, moisture, heat, salt, pollutants and other factors.
This can result in loss of gloss, fading, fine cracks, chalking, blistering, peeling and rust.

As chalking progresses and the film becomes powdery, its ability to protect the substrate drops significantly.
Leaving it untreated at that stage can lead to substrate corrosion and higher repair costs.

Even highly durable coatings do not last forever.
Outdoor products, corrosion-protected products and equipment parts need to be considered together with inspection, repair and recoating.

Recoating is not simply a way to restore appearance.
It is preventive maintenance for protecting the substrate.

The perspective CoatureVietnam values

CoatureVietnam is more than a company that sells coating materials.

We value the correct use of coating materials on the shop floor, the formation of consistent coating films and the protection of customers’ products and equipment.

This requires looking not only at the coating material, but also at the substrate, surface preparation, thinning, film thickness, drying, equipment, operators, inspection and service environment.

Coating quality is not determined by a single material.
It appears in the coating film as the result of the entire process.

That is why we value thinking from the coating film, rather than just the coating material.

Thinking from the film helps identify coating defect causes more accurately.
It helps create standards that allow operators to work without uncertainty.
It helps contribute to customers’ quality consistency, productivity and working environment.

Coating the Future with Science and Care.
This is a way of thinking that improves the future of the shop floor and the customer through the coating film.

Related topics

• Coating material is not just “colored liquid” — The basics of resin, pigments, additives and solvents
• Coatings that dry and coatings that solidify through reaction — The difference between drying and curing
• A hard coating film is not always better — Consider film strength from the application
• Poor adhesion is not just a coating material problem — Examining substrates, surface preparation and wettability
• Do not manage by viscosity alone — The basics of thinning ratio, solids content and film thickness
• Select coating materials from the application, not just the coating name
• How to interpret test data correctly — Specification values alone cannot solve shop-floor problems
• Do not look only to coating materials for the causes of coating defects
• Coating films have a service life — Degradation, chalking and recoating as preventive maintenance