Do not think of a coating film as a single layer

When we look at a coated product, what we usually see is the color of its outermost layer.

Red, black, white, silver. Glossy or matte. An attractive finish or a slightly uneven one.

As a result, when a coating problem occurs, attention tends to focus on the topcoat paint alone.

“Could the topcoat color be wrong?” “Could the topcoat lack gloss?” “Would changing the topcoat paint solve it?”

The topcoat is certainly important. However, coating quality is not determined by the topcoat alone.

For products requiring appearance quality or durability, such as automobiles, construction machinery, home appliances, building materials, and steel structures, the coating film may consist of several layers.

Primer. Intermediate coat. Topcoat.

Each layer has a different role.

In other words, a coating film should be viewed as a quality system made up of several layers with distinct roles, rather than a single film.

Why is the coating film divided into several layers?

Coatings serve many purposes.

These include the following:

• Adhere firmly to the substrate
• Prevent rust
• Level irregularities in the substrate
• Build coating film thickness
• Withstand stone chipping and impact
• Limit the passage of moisture and oxygen
• Provide color
• Provide gloss
• Resist ultraviolet light
• Resist soiling
• Maintain appearance over a long period

It is difficult for one paint to meet all of these requirements.

The coating film is therefore divided into several layers, each with its own role.

The primer is the foundation that protects the substrate. The intermediate coat links performance and appearance. The topcoat is the surface layer responsible for the final appearance and weather resistance.

Understanding this division of roles changes how we approach coating problems.

The primer’s role: an unseen foundation for quality

The primer is hardly visible in the finished product.

Nevertheless, it is a very important layer for coating film quality.

The main roles of the primer are as follows:

• Ensure adhesion between the substrate and coating film
• Prevent rust
• Protect the substrate surface
• Provide a foundation for subsequent coating layers
• Reduce the substrate’s influence on the topcoat

In metal coating in particular, the primer has a major influence on rust prevention.

Iron rusts when exposed to moisture and oxygen. Proper surface preparation and application of a rust-preventive primer at the appropriate film thickness are therefore important.

An inadequate primer can allow rust to appear later, even if the initial finish looks attractive. Poor adhesion to the substrate can also lead to peeling or blistering.

The primer is not unimportant simply because it cannot be seen. It is a layer that must be controlled precisely because it is hidden.

Problems caused by primer defects

Problems with the primer can cause the following defects:

• Rust
• Blistering
• Peeling
• Poor adhesion
• Topcoat discoloration
• Reduced durability of the entire coating film

For example, even an attractive topcoat will not prevent moisture or salt from causing rust if the primer provides insufficient rust protection.

If inadequate surface preparation leaves oil, rust, or dust behind, the primer will not adhere sufficiently to the substrate. The entire coating film may then detach or peel.

When rust or peeling occurs, do not examine the topcoat alone.

The following points must be checked:

• Was the substrate surface clean?
• Was any oil or rust left behind?
• Was surface preparation appropriate?
• Was the primer film thickness sufficient?
• Were the primer drying requirements followed?
• Was there any compatibility problem with the topcoat?

The primer is the foundation of quality. If that foundation is weak, long-term quality will remain unstable no matter how good the paint applied over it is.

The intermediate coat’s role: linking the primer and topcoat

The intermediate coat is the layer between the primer and topcoat.

Not every coating specification requires an intermediate coat. However, it has an important role when high appearance quality or durability is required.

The main roles of the intermediate coat are as follows:

• Level irregularities in the underlying surface
• Improve the topcoat finish
• Build coating film thickness
• Improve resistance to impact and stone chipping
• Slow the penetration of moisture and oxygen
• Stabilize topcoat color development and gloss

In automotive coating, the intermediate coat provides the foundation for topcoat appearance. If the intermediate coat is not smooth, even a well-applied topcoat cannot achieve sufficient appearance quality.

In corrosion-protection coating for steel structures, the intermediate coat may act as a barrier layer. Making it harder for moisture and oxygen to reach the substrate slows corrosion.

In other words, the intermediate coat is the layer that links the primer’s protective function with the topcoat’s appearance function.

Problems caused by intermediate coat defects

Problems with the intermediate coat can cause the following defects:

• Rough topcoat texture
• Poor gloss
• Uneven color
• Insufficient smoothness
• Insufficient chip resistance
• Insufficient corrosion protection
• Intercoat peeling

For example, poor sanding of the intermediate coat may leave a visibly rough texture after topcoating. An insufficient intermediate coat thickness can reduce resistance to stone chipping or impact. Applying the topcoat before the intermediate coat has dried sufficiently can also cause poor intercoat adhesion or blistering.

The intermediate coat is difficult to see after completion. However, it affects both appearance quality and durability.

When an appearance defect occurs, it is important to check the intermediate coat as well as the topcoat.

The topcoat’s role: determining final appearance and surface performance

The topcoat is the outermost coating film.

It is the visible part of the finished product and the layer that customers and consumers assess directly.

The main roles of the topcoat are as follows:

• Provide color
• Provide gloss
• Enhance decorative appearance
• Resist ultraviolet light
• Protect the coating film from rain and dirt
• Provide weather resistance
• Improve cleanability and stain resistance

For home appliances, automobiles, building materials, equipment exteriors, and similar products, topcoat appearance quality is directly linked to product value.

The color is off. The gloss is different. Dirt is embedded in the coating. The finish is uneven. The surface feels rough.

Customers immediately notice these defects.

For topcoating, application conditions, drying conditions, the booth environment, operator skills, and inspection criteria are therefore important alongside paint performance.

Problems caused by topcoat defects

Problems with the topcoat can cause the following defects:

• Color difference
• Poor gloss
• Dirt and inclusions
• Sagging
• Roughness
• Orange peel
• Blushing
• Discoloration
• Insufficient weather resistance
• Poor stain resistance

However, even a defect visible in the topcoat does not necessarily originate in the topcoat paint alone.

For example, poor gloss may result from a rough underlying surface or insufficient drying of the intermediate coat rather than the topcoat paint. Uneven color may result from uneven film thickness or thinner selection as well as topcoat application technique. Dirt defects may originate in the booth environment, work clothing, air lines, or product storage rather than the paint.

The topcoat is the most visible layer. That makes it especially important not to restrict the investigation of causes to that layer alone.

The importance of intercoat adhesion

In a multilayer coating film, the layers must adhere firmly to one another.

Substrate and primer. Primer and intermediate coat. Intermediate coat and topcoat.

Insufficient adhesion at any of these interfaces can lead to peeling or blistering.

The following factors affect intercoat adhesion:

• Insufficient drying of the preceding layer
• Recoating intervals that are too short or too long
• Surface contamination
• Residual sanding dust
• Oil contamination
• Poor compatibility between paints
• Unsuitable baking conditions
• Excessive film thickness
• Reduced adhesion due to excessive curing

Recoating either too early or too late can cause problems.

Recoating too early leaves solvent or moisture in the underlying layer, which can cause blistering or poor drying. Recoating too late may allow the underlying layer to cure excessively, reducing adhesion to the next layer.

Recoating intervals must therefore be controlled as standard conditions, rather than left to judgment on the shop floor.

A thicker coating film is not always better

Film thickness control is important in coating.

Insufficient film thickness causes problems such as the following:

• Insufficient rust protection
• Insufficient hiding power
• Insufficient weather resistance
• Insufficient abrasion resistance
• Uneven appearance
• Premature deterioration

Excessive film thickness also causes problems:

• Sagging
• Poor drying
• Insufficient curing
• Solvent popping
• Cracking
• Poor intercoat adhesion
• Increased costs
• Dimensional defects

In other words, greater thickness does not automatically provide greater assurance. What matters is consistently achieving the required film thickness in the required locations.

On products with complex shapes in particular, film thickness varies between flat areas, edges, recesses, and areas around holes.

Measuring film thickness only on easily accessible flat areas is insufficient. Also check edges, recesses, welds, rear surfaces, lower areas, and other locations where defects are likely to occur.

Surface preparation: the foundation beneath the coating system

When discussing primers, intermediate coats, and topcoats, we tend to focus only on the coating layers.

However, an important process lies beneath the coating film itself.

That process is surface preparation.

Surface preparation brings the substrate surface into a suitable condition before coating.

For metals, it includes degreasing, rust removal, chemical conversion treatment, and blasting. For plastics, it includes release-agent removal, surface modification, and primer treatment. For wood or concrete, moisture, absorption, alkalinity, and surface strength must be checked.

With poor surface preparation, the coating film cannot deliver its full performance, however good the paint may be.

Poor adhesion. Rust. Blistering. Peeling. Discoloration.

In many cases, surface preparation is the actual cause of these defects.

The coating film is formed on the substrate surface. If that surface has not been properly prepared, the entire coating film becomes unstable.

Identify the cause by asking which layer has the problem

When a coating problem occurs, first clarify the defect observed.

Then consider which layer is likely to be responsible.

When rust appears

Check the primer and surface preparation.

• Was any rust left on the substrate?
• Was degreasing sufficient?
• Is the rust-preventive primer appropriate?
• Was the primer film thickness sufficient?
• Is rust developing from edges or scratches?

When peeling occurs

Check the interface where separation is occurring.

• Is it between the substrate and primer?
• Is it between the primer and intermediate coat?
• Is it between the intermediate coat and topcoat?
• Is the recoating interval appropriate?
• Was there dirt or sanding dust on the surface?

When poor gloss occurs

The topcoat is not the only layer to check.

• Is the topcoat film thickness appropriate?
• Is the thinner’s drying rate suitable?
• Is the intermediate coat surface smooth?
• Are the drying conditions appropriate?
• Is there a problem with the booth environment?

When dirt or inclusions occur

Paint is not the only item to check.

• Is paint filtration sufficient?
• How clean is the booth?
• Has dust settled on products during storage?
• Are work clothing or air blow-off operations introducing foreign matter?
• Is foreign matter being carried in from the preceding process?

Breaking down the problem layer by layer in this way improves the precision of corrective action.

Common oversights concerning coating layers in Vietnamese factories

In my experience, the following oversights are common in coating operations in Vietnam:

• Examining only the topcoat, without checking the primer or surface preparation
• Measuring film thickness only on flat areas
• Leaving recoating intervals to individual operators
• Failing to control the waiting time after surface preparation
• Insufficient dust removal after sanding
• Applying the topcoat before the primer has dried sufficiently
• Allowing scratches or contamination during storage after coating
• Failing to standardize repair-coating conditions

Under these circumstances, changing the paint will not provide a fundamental improvement.

To reduce coating problems, each layer and its process conditions must be controlled through standard work procedures.

CoatureVietnam’s 4M perspective

Organizing the checks using the 4Ms is an effective way to stabilize the coating system.

Material: materials

• Is the primer, intermediate coat, and topcoat combination appropriate?
• Are preparation chemicals suited to the substrate?
• Is the thinner suited to the required drying behavior and surface texture?
• Has compatibility between the paints been confirmed?
• Are repair paints selected using the same principles?

Method: methods

• Are surface preparation conditions standardized?
• Is the required film thickness specified for each layer?
• Are recoating intervals controlled?
• Are drying requirements followed?
• Are sanding, cleaning, and repair methods clearly defined?

Machine: equipment

• Does preparation equipment function sufficiently?
• Are booth airflow and cleanliness stable?
• Is the temperature distribution in the drying oven appropriate?
• Are film thickness gauges and inspection equipment available?
• Are fixtures suited to the product shape being used?

Man: people

• Do operators understand the role of each layer?
• Are they giving the primer sufficient attention?
• Are they aware of the need to follow recoating intervals?
• When a defect occurs, can they consider which layer has the problem?
• Do managers communicate inspection criteria clearly?

The Management perspective is also important.

Without specifications, SOPs, inspection criteria, training materials, and rules to prevent recurrence, efforts on the shop floor alone cannot stabilize quality.

Questions for customer visits

The following questions are useful when investigating problems with the coating system:

• How many coating layers does this product’s specification require?
• Are the roles of the primer, intermediate coat, and topcoat clear?
• Are film thickness standards specified for each layer?
• Are recoating intervals controlled?
• How long after surface preparation is the coating applied?
• Are the drying requirements after priming followed?
• When a topcoat defect occurs, do you also check the intermediate coat and underlying surface?
• When peeling occurs, do you identify the interface where separation is taking place?
• Do film thickness measurements cover edges and recesses as well as flat areas?
• Are there standard conditions for repair coating?

These are practical checks that sales team members can also use easily.

Even when a customer believes the paint is at fault, these questions can broaden the investigation to surface preparation, film thickness, drying, recoating, and storage conditions.

Conclusion: coating film quality begins with understanding each layer’s role

A coating film is not simply one colored layer.

Especially where high coating quality is required, the primer, intermediate coat, and topcoat each have their own role.

The primer provides the foundation for adhesion to the substrate and rust prevention. The intermediate coat supports appearance and durability. The topcoat provides the final appearance and surface performance.

To prevent coating problems, do not judge the system by the visible topcoat alone.

Checks must include surface preparation, the primer, intermediate coat, topcoat, film thickness, drying, recoating intervals, and intercoat adhesion.

CoatureVietnam views coating as a process that protects product value, rather than merely an operation.

Coating the Future with Science and Care.

The first step in putting this into practice is to understand the function of each layer in the coating film.