Absorption of water or solvent reduces adhesion as well as affecting appearance

A coating film may swell after application.

The surface rises.
Local areas bulge.
The film becomes soft.
Abnormalities appear only where water has contacted it.
In high humidity, the film turns whitish and swells.
The film swells after contact with chemicals or cleaning agents.

This phenomenon is called swelling.

Swelling occurs when a film absorbs water, moisture, solvents, chemicals, or similar substances and increases in volume.

Visually, it appears as bulges or raised areas.
However, swelling is more than an appearance defect.

A swelling film may indicate that water or solvent has entered the film and destabilized its structure.
This can lead to reduced adhesion, peeling, reduced corrosion protection, or film softening.

Swelling therefore needs to be viewed as a sign that the film cannot adequately withstand its operating environment.

1. What is swelling?

Swelling occurs when a film takes in water, solvent, or similar substances from outside and expands.

A coating film is not a perfectly impermeable sheet of glass that excludes all external substances.
Depending on the coating type, film thickness, curing condition, and operating environment, water or solvent may enter the film.

Once water or solvent enters, the resin forming the film may soften or increase in volume.
The film then swells.

As swelling progresses, it also affects the interface between the film and substrate.
The interface is where the substrate and film meet.
Water entering this area reduces adhesion and leads to blistering or peeling.

Swelling involves the film interior and substrate interface, rather than only the film surface.

2. Main factors causing swelling

Swelling has more than one cause.

Typical factors include the following.

• High humidity
• Water contact
• Condensation
• Insufficient drying after water washing
• Rainwater
• Outdoor storage
• Chemical contact
• Contact with cleaning agents
• Solvent contact
• Insufficient film curing
• Insufficient film thickness
• Insufficient water resistance of the coating
• Insufficient chemical resistance of the coating
• Insufficient surface preparation
• Moisture in the substrate
• Deterioration of the old film
• Water ingress through pinholes or scratches

Particular care is required if the film contacts water or solvent before it is fully dried and cured.

Even if the surface is dry, insufficient internal curing makes the film more susceptible to water or solvent.

3. Why moisture and water enter the film

The film protects the substrate.
However, water may gradually enter depending on the film’s type and condition.

Hot, humid environments particularly increase susceptibility to water.

High humidity continues.
Condensation occurs.
Drying after water washing is insufficient.
The film is exposed to rainwater.
Water enters through scratches or pinholes.

These conditions make water more likely to enter the film interior or the substrate interface.

Water ingress can soften or swell the film.
If the substrate is metal, water reaching the interface can also cause rust.

Swelling is therefore related to reduced corrosion protection as well as appearance defects.

4. Insufficient drying or curing increases the likelihood of swelling

Swelling is more likely when the film is not sufficiently dried or cured.

An insufficiently dried film may contain residual solvent or inadequately cured resin.
Its structure is not yet stable.

Contact with water or solvent then softens it further and makes swelling more likely.

The following conditions require particular care.

• Proceeding to the next process based only on dry-to-touch condition
• Insufficient baking temperature
• Insufficient workpiece temperature
• Insufficient curing time for a two-component coating
• Incorrect hardener proportion
• Delayed internal drying caused by excessive film thickness
• Water contact immediately after coating
• Moisture trapped inside packaging

Preventing swelling requires more than selecting a highly water-resistant coating. Check that the film is dried and cured sufficiently to develop its intended performance.

5. Moisture in the underlying surface also causes swelling

Swelling can result from moisture in the underlying surface as well as water entering from outside the film.

For example, wood and porous materials may contain moisture.
Even on water-washed metal parts, insufficient drying can leave water in gaps or recesses.
Coating over residual water after surface preparation traps water beneath the film.

If that water later moves as vapor or moisture, it pushes the film upward, causing blistering or swelling.

Check the following points.

• Is drying after water washing sufficient?
• Is water left in recesses or joints?
• Is wood moisture content appropriate?
• Is water left in porous materials?
• Are oven conditions appropriate for drying after surface preparation?
• Is condensation present before coating?

Water present before coating appears as defects afterward.
Checking drying before coating is important for preventing swelling.

6. Swelling caused by chemicals, cleaning agents, or solvents

Chemicals and solvents cause swelling as well as water.

Contact with chemicals or cleaning agents may affect the film’s resin, making it soften or swell.

The following environments require particular care.

• Products wiped with cleaning agents
• Products cleaned with alcohol or solvents
• Parts exposed to acids or alkalis
• Parts exposed to cutting oils or lubricants
• Equipment exposed to chemical mist inside a factory
• Parts chemically cleaned in food, medical, or electronics applications
• Parts exposed outdoors to acid rain or pollutants

When selecting a coating material, check what the product may contact, as well as its color and appearance.

Will it “only contact water”?
Will it “be wiped daily with a cleaning agent”?
Will it “be immersed in chemicals”?
Will it “have oil on it”?

The operating environment changes the required film performance.

7. The relationship between swelling, blistering, and peeling

Progressive swelling can lead to blistering or peeling.

The film absorbs water or solvent and swells.
Pressure develops inside the film.
Water enters the substrate interface.
Adhesion decreases.
The film lifts away from the substrate.
Eventually, it peels off.

Defects progress through this sequence.

In metal coating in particular, water beneath the film may cause rust.
Rust increases in volume and pushes the film up further.
Blistering and peeling then spread.

Detect swelling at an early stage.
Mild swelling may sometimes be improved by reviewing the operating environment or drying conditions.
However, if adhesion loss or rust has progressed, stripping and recoating may be necessary.

8. Points requiring particular attention in Vietnamese factories

Several conditions in Vietnamese factories make swelling more likely.

First, high temperature and humidity.
Films and substrates are more susceptible to moisture in high-humidity environments.

Next, the rainy season.
Moisture increases in coating areas, storage areas, and during transport.
Rainwater and condensation also require attention if products are temporarily stored outdoors.

Insufficient drying after waterborne surface preparation is also important.
Inadequate drying after washing, degreasing, or chemical conversion treatment may trap water beneath the film.

The use of chemicals and cleaning agents must also be checked.
Factories sometimes wipe products with alcohol, thinner, or cleaning agents.
If the chemical is incompatible with the film, swelling or gloss changes may occur.

In Vietnam, check humidity, water, cleaning agents, and storage conditions together.

9. Shop-floor checkpoints

If swelling occurs, check the following items.

Occurrence conditions

• Is the film swollen?
• Has the surface softened?
• Does it occur only where water has contacted the film?
• Does it occur only where chemicals have contacted the film?
• Does it affect the whole surface or only local areas?
• Has it progressed to blistering or peeling?
• Is rust present?

Operating environment

• Is the product used in high humidity?
• Is it used outdoors?
• Does it contact rainwater?
• Does condensation occur?
• Is it washed with water?
• Is it wiped with cleaning agents or chemicals?
• Does it contact oil or solvent?
• Is it stored in hot, humid conditions?

Application conditions

• Is film thickness appropriate?
• Are there pinholes or scratches?
• Are drying and curing sufficient?
• Is baking temperature insufficient?
• Is workpiece temperature checked?
• Is the hardener proportion accurate in the two-component coating?
• Does the film contact water immediately after application?

Underlying surface conditions

• Is drying after surface preparation sufficient?
• Is water left after washing?
• Is moisture content in wood or porous materials appropriate?
• Is oil or salt left on the substrate?
• Has the old film or primer deteriorated?
• Is rust present?

Material conditions

• Is the coating’s water resistance sufficient?
• Does chemical resistance meet the requirements?
• Is the coating system suitable for the operating environment?
• Is the primer and topcoat combination appropriate?
• Is a sealer or primer needed?

These checks help distinguish whether swelling originates from water, chemicals, insufficient drying, or the underlying surface.

10. Improvement perspectives CoatureVietnam can offer

CoatureVietnam does not view swelling simply as a bulging film.
We organize it from the 4M perspectives as a phenomenon in which water, solvent, or chemicals enter the film.

Material

• Is the coating’s water resistance sufficient?
• Does chemical resistance suit the operating environment?
• Is the primer or sealer appropriate?
• Is the primer and topcoat combination appropriate?
• Is the substrate prone to retaining moisture?

Method

• Is drying after surface preparation sufficient?
• Is condensation present before coating?
• Is film thickness appropriate?
• Is drying and curing time after application sufficient?
• Has conditioning time before water or chemical contact been defined?

Machine

• Is the oven for drying after surface preparation adequate?
• Is workpiece temperature in the drying oven appropriate?
• Is storage-area humidity controlled?
• Are air blow-off and drying equipment after washing appropriate?
• Is there environmental control to prevent condensation?

Man: Operators

• Do operators understand the risks of residual water?
• Are they allowing water or chemical contact immediately after coating?
• Do they understand compatibility between cleaning agents and films?
• Can they assess the relationship between swelling, blistering, and peeling?

From the Management perspective, establish post-preparation drying standards, conditioning times after coating, operating-environment checks, cleaning-agent use rules, checks of water and chemical resistance requirements, and response criteria when swelling occurs.

Conclusion

Swelling occurs when a film absorbs water, moisture, solvents, chemicals, or similar substances and expands.

It is more than an appearance defect.
Water or solvent entering the film or substrate interface can lead to reduced adhesion, blistering, peeling, or reduced corrosion protection.

Preventing swelling requires checks of drying after surface preparation, substrate moisture, film curing, film thickness, the operating environment, cleaning agents, and storage conditions, as well as the coating’s water and chemical resistance.

In hot, humid environments such as Vietnam, particular care is needed with moisture, insufficient drying after washing, condensation, outdoor storage, and chemical contact.

Coating the Future with Science and Care
CoatureVietnam views swelling as water or solvent entering the film, comprehensively checks materials, processes, drying, and the operating environment, and supports coating quality that is less prone to recurrence.