Assessing the substrate, surface preparation, and wettability

Poor adhesion is one of the coating problems frequently raised in consultations.

The coating film peels off. The film comes off in a tape test. The film lifts when the product is bent. The film peels back from the edges. The film delaminates during use.

When these defects occur, the first suspicion is often that something is wrong with the paint.

Of course, the selected paint may be unsuitable. Using a paint that is inappropriate for the substrate can cause poor adhesion.

However, poor adhesion is not determined by the paint alone.

Adhesion occurs between the paint and the substrate surface. It is therefore necessary to consider not only the paint but also the substrate, surface preparation, surface condition, wettability, film thickness, and drying and curing conditions.

Adhesion means the coating film is firmly attached to the substrate surface

Put simply, adhesion is the state in which the coating film is firmly attached to the substrate surface.

In practice, however, it is not quite that simple.

Paint is applied to the substrate surface, then dries or cures to become a coating film. Stable adhesion cannot be achieved unless the paint spreads sufficiently over the surface and makes thorough contact with the substrate.

Oil, moisture, rust, dust, mold release agents, sanding dust, or other residues on the substrate surface create a weak layer between the film and the substrate.

When this weak layer exists, even a good paint cannot adhere directly to the substrate. Peeling can consequently occur during tape testing, bending, impact, or exposure to the service environment.

When investigating poor adhesion, it is therefore necessary to consider not only the paint but also whether the paint is making proper contact with the substrate surface.

Only one side of the coating film touches the substrate

A coating film has a distinctive position.

One side touches the substrate. The other side is exposed to the air or the service environment.

The coating film therefore has two roles.

One is to adhere firmly to the substrate. The other is to protect the substrate from the external environment.

It must fulfill both roles at the same time.

For example, even a film with excellent weather or chemical resistance cannot fulfill its role if adhesion to the substrate is weak. Conversely, even when initial adhesion is good, moisture, heat, chemicals, vibration, and other exposure during use can reduce adhesion.

Adhesion must be maintained not only immediately after coating but also throughout exposure to the service environment.

A clean substrate surface is a prerequisite for adhesion

The first point to check when considering adhesion is whether the substrate surface is clean.

The substrate surface is the surface of the material being coated.

The following residues on the surface before coating can cause poor adhesion.

• Oil
• Moisture
• Rust
• Oxides
• Dust
• Sanding dust
• Mold release agents
• Oil from hands
• Cleaning agent residue
• Contamination from the preceding process

These substances come between the coating film and the substrate, creating a weak interface.

When poor adhesion occurs, the paint name is not the first thing to check. First, verify whether the substrate surface was actually suitable for coating before application.

Paint does not spread over a substrate with poor wettability

Wettability is closely related to adhesion.

Wettability describes how readily paint spreads over the substrate surface.

For example, water placed on an oily surface may be repelled and form droplets. In this condition, the water is not thoroughly wetting the surface.

The same phenomenon occurs with paint.

If the substrate surface repels the paint, the paint will not spread evenly. This can lead to cissing, cratering, poor adhesion, and appearance defects.

Good adhesion requires the paint to wet the substrate surface properly and make sufficient contact with it.

Relevant factors include substrate surface cleanliness, surface energy, surface preparation, primer, paint resin formulation, and thinner selection.

Changing the substrate changes adhesion

Even when the same paint is used, adhesion changes if the substrate changes.

Typical substrates include the following.

• Iron
• Aluminum
• Stainless steel
• Galvanized steel sheet
• Plastic
• Rubber
• Glass
• Wood

Each has different surface properties.

Iron and aluminum require different surface preparation. Stainless steel has a stable surface, which can make adhesion difficult. With galvanized steel sheet, surface treatment and oxidation affect adhesion. Glass and some plastics have smooth surfaces to which paint may not readily adhere.

When poor adhesion occurs, it is therefore necessary to check the substrate first.

Even if you assume that your usual paint will work, a change in substrate can change the result.

Do not assess plastic coating by the material name alone

Plastic coating requires particular care.

Unlike metals, some plastics have low surface energy. This can make it difficult for paint to spread over the surface and achieve adhesion.

Even with the same resin name, coatability varies with the manufacturer, grade, additives, and molding conditions.

For example, materials all called ABS cannot necessarily be coated in the same way. Adhesion to PP also varies depending on surface treatment, additives, and mold release agents used during molding.

Plastic coating must not be assessed by the material name alone.

Checks should cover points such as the following.

• Resin name
• Manufacturer
• Grade
• Additives
• Presence of mold release agents
• Molding conditions
• Presence of surface treatment
• Adhesion testing on the actual molded product

Rather than simply assuming that ABS will be fine or that PP is impossible to coat, it is important to check the actual substrate surface.

Blasting and sanding do more than roughen a surface

Blasting or sanding is sometimes used to improve adhesion in metal coating.

Roughening the surface is understood to give the coating film something to grip. This is one of the effects.

However, blasting and sanding have other purposes as well.

More importantly, they remove surface contamination, rust, oxides, adsorbed water, and other substances to expose a fresh substrate surface.

A fresh substrate surface is favorable for paint adhesion. However, this condition may not last long.

If the product is left for a long time after blasting or sanding, airborne moisture, oil, dust, oxides, and other substances can again contaminate the surface. This can degrade the surface condition achieved through preparation and cause poor adhesion.

It is therefore important to coat as soon as possible after surface preparation.

Controlling time after surface preparation is also important

Preventing poor adhesion requires control of both surface preparation itself and the time between preparation and coating.

For example, immediately after blasting, a surface may be suitable for coating. However, leaving it for a long time in a humid place after treatment changes its condition.

The same applies after degreasing.

Even if oil has been removed immediately after degreasing, subsequent transport, handling, storage, or dust deposition can recontaminate the surface.

Thus, even when surface preparation is performed correctly, recontamination before coating can cause poor adhesion.

The following items should be controlled.

• The maximum number of hours between surface preparation and coating
• Storage location after surface preparation
• Humidity control
• Dust control
• Use of gloves
• Contact during transport
• Prevention of recontamination
• Surface inspection methods

To stabilize adhesion quality, surface preparation and coating need to be managed as one continuous sequence.

Drying and curing conditions also affect adhesion

Poor adhesion is not determined solely before coating. Drying and curing conditions after application also affect adhesion.

For example, an incorrect mixing ratio in a two-component paint may cause insufficient curing and reduced adhesion. If actual workpiece temperature is insufficient for a baking paint, the film may not develop adequate performance. Excessive film thickness can lead to insufficient internal drying or curing and subsequent peeling.

Trapped moisture or solvent before drying can also cause blistering and poor adhesion.

Poor adhesion can therefore arise not only from problems before application but also from inadequate film formation after application.

Organize the causes of poor adhesion using the 4Ms

CoatureVietnam emphasizes organizing the causes of poor adhesion from the perspective of the 4Ms.

Material: materials

Check the paint, thinner, hardener, surface preparation chemicals, and substrate.

• Is the paint suitable for the substrate?
• Is the specified thinner being used?
• Is the hardener correct?
• Are the surface preparation chemicals appropriate?
• Has the substrate manufacturer or grade changed?
• Are mold release agents or oil present on the substrate surface?

Method: methods

Check surface preparation, dilution ratio, film thickness, drying conditions, and application procedures.

• Is the degreasing method standardized?
• How long after surface preparation is coating performed?
• Is the dilution ratio appropriate?
• Is the film too thick?
• Are drying and curing requirements followed?
• Is the flash-off time appropriate?

Machine: equipment

Check surface preparation equipment, coating equipment, drying ovens, and measuring instruments.

• Are there any abnormalities in the surface preparation equipment?
• Are spray conditions stable?
• Is the drying oven’s temperature distribution satisfactory?
• Is actual workpiece temperature checked?
• Are film thickness gauges and other measuring instruments properly managed?

Man: people

Check operator procedures, training, and inspection criteria.

• Do degreasing methods differ between operators?
• Are glove-use and handling rules followed?
• Are parts touched with bare hands after surface preparation?
• Are inspectors using consistent acceptance criteria?
• Are records kept when defects occur?

Poor adhesion can result from several overlapping factors rather than a single factor.

Points to check when poor adhesion occurs

Checking the following items in order makes it easier to identify the causes of poor adhesion.

• Does the defect occur immediately after application, after drying, or after use?
• Does the defect affect the entire surface, particular areas, or the edges?
• Has the substrate changed?
• Has the substrate manufacturer or grade changed?
• Is there oil, moisture, rust, dust, or mold release agent on the surface?
• Is the degreasing method appropriate?
• How much time elapsed between surface preparation and coating?
• Could recontamination have occurred after surface preparation?
• Is the paint wetting the substrate surface?
• Is cissing or cratering occurring?
• Are the dilution ratio, film thickness, and drying conditions appropriate?
• For a two-component paint, are the mixing ratio and pot life requirements followed?
• For a baking paint, is actual workpiece temperature checked?
• Has the same defect occurred before?

These checks make it easier to decide whether to change the paint, improve surface preparation, or revise process controls.

Changing the paint alone may not prevent recurrence

Changing the paint is one option when poor adhesion occurs. However, it may not solve the problem by itself.

For example, if insufficient degreasing is the cause, the problem may recur even after a paint change. If excessive waiting time after blasting is the cause, changing the paint will not address the root cause. If the substrate grade has changed, the substrate needs to be investigated. If insufficient baking is the cause, drying conditions need to be reviewed rather than the paint.

Changing the paint can be effective in some cases. However, changing only the paint without checking process-related causes can also create other defects.

Before changing the paint in response to poor adhesion, first examine the surface. Examine the substrate. Examine surface preparation. Examine drying conditions.

This order matters.

How CoatureVietnam can help

CoatureVietnam does not treat poor adhesion as a paint problem alone.

We identify causes by considering the substrate, surface preparation, wettability, dilution ratio, film thickness, and drying conditions together.

For example, we can provide the following support.

Paint Optimizer We review the conditions under which the existing paint is used and investigate process-related causes of poor adhesion.

Paint Voyager We consider candidate paints suited to the substrate and application and define primers and coating specifications as necessary.

Local Optimer When considering local alternative paints, we check not only color but also adhesion to the actual substrate.

Work standards and process control We establish SOPs for degreasing, time control after surface preparation, film thickness, drying conditions, and inspection criteria to help prevent recurrence.

Coat Visioner We organize the conditions, locations, lots, and process data associated with poor adhesion to make defect trends visible.

Summary

Poor adhesion is not just a paint problem.

Adhesion is determined by the relationship between the paint and the substrate surface. The substrate, surface condition, surface preparation, wettability, and drying and curing conditions therefore need to be assessed together.

Even good paint will not adhere to a contaminated substrate surface. Even good surface preparation will not ensure stable adhesion if recontamination occurs before coating. Materials with the same name may have different adhesion characteristics if their manufacturers or grades differ. Even a dry surface will not deliver the required coating film performance if curing is insufficient.

When poor adhesion occurs, it is important to examine the whole process first rather than immediately suspecting the paint.

Coating the Future with Science and Care. CoatureVietnam does not dismiss poor adhesion as a paint problem. We help develop coating processes less prone to recurrence by considering the substrate, surface preparation, application conditions, and drying conditions together.