A Hard Coating Film Is Not Always a Good Coating Film

Fine cracks sometimes develop in a coating film after application.

Linear cracks appear on the surface.
Cracks appear at corners or bends.
The film cracks after some time.
Fine cracks increase after outdoor use.
The film cracks on wood or plastic components.
Cracks form in thickly coated areas.

These defects are referred to as cracking or cracks.

When cracks appear, people tend to assume a weak film, defective paint, or poor drying.
These can certainly be causes.

However, the essential nature of cracking extends beyond them.

In many cases, cracks develop when substrate movement and coating-film movement no longer match.

Substrates expand and contract with temperature, humidity, loading, processing, and changes over time.
Coating-film hardness and flexibility also change through drying, curing, and deterioration.

The substrate moves.
The coating film cannot accommodate that movement.
The film then cracks.

This is an important basic principle for understanding cracking.

1. What Is Cracking?

Cracking is a defect in which fractures form in the coating film.

The crack pattern varies according to its cause.

Fine linear cracks.
Network-like cracks.
Cracks along corners.
Cracks at bends.
Cracks in thick-film areas.
Cracks confined to the film surface.
Deep cracks reaching the substrate.

Similar-looking cracks do not necessarily have the same cause.

Only the surface may be cracked.
The entire film may be cracked.
Cracks may follow substrate movement.
Drying shrinkage may cause cracks.
Aging may embrittle the film and cause cracks.

When cracks are found, first check the following.

• Where do they occur?
• When did they occur?
• Are they confined to the surface or do they reach the substrate?
• Are they concentrated at bends, corners, or thick-film areas?
• Are they related to substrate movement?
• Did they appear after outdoor use?

Examining crack patterns and onset timing helps isolate causes.

2. Substrates Move

A coated product’s substrate may look stationary.
In reality, it moves.

Metals expand and contract with temperature.
Plastics deform or change dimensions under heat or load.
Wood expands and contracts with humidity.
Concrete and building materials may also move with moisture or temperature.

A substrate is therefore not completely motionless after coating.

Small substrate movements are less likely to cause problems if the coating film can flexibly accommodate them.
However, an excessively hard or thick film, an embrittled film, or weak adhesion may prevent it from withstanding movement and cause cracking.

The following substrates require particular attention.

• Wood
• Molded plastic parts
• Thin sheet metal
• Aluminum
• Galvanized steel sheet
• Bent components
• Components exposed to temperature changes
• Outdoor products
• Components used in humid environments

Preventing cracking requires considering how the substrate moves, as well as the coating film.

3. A Hard Coating Film Is Not Always Better

Coating films need hardness.

Scratch resistance.
Abrasion resistance.
Chemical resistance.
Durability.
Appearance retention.

A hard film can be beneficial for these properties.

However, greater hardness is not necessarily better.

An excessively hard film struggles to accommodate substrate expansion, contraction, or bending.
Stress may then concentrate and cause cracks.

Film flexibility is particularly important for plastic parts, wood, thin sheet, bends, and outdoor components.

A coating film needs more than hardness alone.
It needs a balance of hardness and flexibility suited to the service environment.

For example, outdoor components exposed to temperature changes require some ability to accommodate movement.
Materials such as wood that move with humidity need a film designed to withstand expansion and contraction.
Plastic components require a film that can accommodate flexing or thermal deformation.

Do not assess coating-film performance by hardness alone.

4. Excessive Film Build Can Cause Cracking

Excessive film thickness can also cause cracking.

A thick film may develop greater internal stress during drying and curing.
Internal stress is the tensile or shrinkage force remaining within the film.

As paint dries and cures, solvent leaves and the resin solidifies.
The film shrinks in this process.
Thick films are more prone to differences in surface and internal drying rates, making residual stress more likely.

The film may crack as a result.

The following conditions require particular attention.

• Thick application in one pass
• High total thickness of primer, intermediate coat, and topcoat
• Thick filler or surfacer
• Paint accumulating at corners or recesses
• Overly rapid drying
• Excessively high baking temperature
• Thick application of a hard paint

Achieving the required film thickness is important.
However, excessive build causes cracking, pinholes, foaming, poor drying, or after-tack.

Greater film thickness is not necessarily better.
Control it uniformly within the appropriate range.

5. Drying and Baking Conditions Also Affect Cracking

Drying and baking conditions also influence cracking.

Overly rapid drying can harden the surface first and increase the difference in shrinkage from the interior.
This leaves internal stress and makes cracking more likely.

Excessive baking temperature can make the film overly hard or brittle.
Whites and light colors may also yellow.

Insufficient drying or underbaking also causes problems.
An inadequately cured film can deform in subsequent processes or develop after-tack or poor adhesion.
Cracks may then appear later.

The following checks are important in drying and baking.

• Is initial heating too rapid?
• Is setting time sufficient?
• Is workpiece temperature appropriate?
• Is baking time appropriate?
• Is the film being overbaked?
• Are only thick-film areas drying slowly?
• Is heat deforming the substrate?

Cracking prevention requires checking drying conditions as well as selecting paint.

6. Moisture Content Matters in Wood

Wood coating has conditions that make cracking more likely.

Wood absorbs and releases moisture with changes in humidity.
It expands when it absorbs moisture and contracts when it dries.

Cracks develop if the film cannot accommodate this movement.

The key point for wood is moisture content.
Moisture content indicates how much water the wood contains.

Coating wood with high moisture content can allow later drying shrinkage to crack the film.
Moisture attempting to escape from inside the wood may also cause blistering or poor adhesion.

Check the following points in wood coating.

• Is moisture content appropriate before application?
• Is dried wood being used?
• Will the product be used outdoors or indoors?
• Does the wood absorb coating uniformly?
• Is a primer or sealer being used?
• Are measures in place to prevent moisture absorption at end grain?
• Is the film flexible?

Wood coating requires controlling moisture in the wood itself, as well as the paint.

7. Examine Flexing and Thermal Deformation in Plastic Components

Cracks can also occur on plastic components.

Plastics are softer than metals and more prone to temperature-related deformation.
Molded parts may also contain residual stress.
The substrate moves when it bends or flexes in use, expands with heat, or experiences loading.

Cracks develop if the coating film cannot accommodate that movement.

The following conditions require particular attention.

• A hard film applied to a soft plastic
• Components that bend or flex
• Use in a high-temperature environment
• Excessive film thickness
• Drying or baking temperatures too high for the plastic
• Residual stress in the substrate
• Solvent cracking caused by solvent

When coating plastics, check how the substrate actually moves in use, as well as its material name.

8. Consider Cracking from Aging in Outdoor Service

Coated products used outdoors may develop cracks over time.

Ultraviolet radiation, heat, oxygen, moisture, rain, chemicals, and salt gradually deteriorate the film.

As deterioration progresses, the film may harden and lose flexibility.
It then becomes unable to accommodate substrate expansion and contraction or temperature changes, and cracks develop.

Check outdoor products for the following signs of deterioration as well.

• Fading
• Yellowing
• Gloss reduction
• Haze
• Chalking
• Powdering
• Reduced adhesion
• Reduced corrosion protection

Cracking does not necessarily occur in isolation.
It may appear as part of coating-film deterioration.

For outdoor applications, select paint based on long-term durability as well as initial appearance.

9. Examine Crack Locations

Where cracks occur is important for isolating the cause.

Cracks at bends suggest substrate deformation or insufficient film flexibility.
Cracks at corners may involve insufficient or excessive thickness, stress concentration, or drying shrinkage.
Cracks in thick-film areas suggest drying shrinkage or internal stress.
Cracks along wood grain may involve wood shrinkage or moisture content.
Fine cracks across an outdoor surface suggest aging or ultraviolet deterioration.

Check the following points.

• Are cracks concentrated at bends?
• Do they appear at corners?
• Do they appear in thickly coated areas?
• Do they follow wood grain or substrate direction?
• Do they appear in repaired areas?
• Did they appear after outdoor use?
• Are they directional or network-like?
• Are they confined to the surface or do they reach the substrate?

Defect locations provide important information about the cause.

10. Points Requiring Particular Attention in Factories in Vietnam

Several conditions in factories in Vietnam can lead to cracking.

First are heat and humidity.
Wood and moisture-absorbing materials experience greater moisture exchange.
Expansion or contraction after application can exceed the film’s ability to accommodate movement and cause cracks.

Next is the outdoor service environment.
Strong sunlight in Vietnam exposes outdoor components to ultraviolet radiation and heat.
Progressive film deterioration leads to cracking.

Excessive film build also requires attention.
Thick application for hiding power or appearance increases drying shrinkage and internal stress.

Another risk is continuing to use the same paint after the substrate changes.
Using metal paint on plastic.
Using a hard film on a soft substrate.
Using indoor paint for outdoor applications.

These practices can cause cracking in service.

Factories in Vietnam need to check the substrate, service environment, film hardness, film thickness, and drying conditions together.

11. Shop-Floor Checkpoints

When cracking occurs, check the following items.

Defect Condition

• Are the cracks linear or network-like?
• Are they confined to the surface or do they reach the substrate?
• Where do they occur?
• Are they concentrated at bends, corners, or thick-film areas?
• When did they occur?
• Was it immediately after application, after drying, or after use?
• Do they increase after outdoor use?

Substrate Conditions

• Is the substrate metal, plastic, or wood?
• Does it expand and contract readily?
• Is it exposed to temperature changes?
• Does it move with humidity?
• For wood, is moisture content appropriate?
• For plastic, is there flexing or thermal deformation?
• Is coating applied after bending?

Coating-Film Conditions

• Is the film too hard?
• Is flexibility sufficient?
• Is film thickness excessive?
• Is total primer and topcoat thickness appropriate?
• Are only thick-film areas cracking?
• Is drying shrinkage excessive?

Drying Conditions

• Is drying too rapid?
• Is baking temperature too high?
• Is workpiece temperature appropriate?
• Is the film being overbaked?
• Is curing sufficient?
• Is setting time appropriate?

Service Environment

• Is the product used outdoors?
• Is it exposed to direct sunlight?
• Is the environment hot?
• Is the environment humid?
• Does it contact chemicals or cleaning agents?
• Are temperature differences large?
• What service life is required?

These checks help identify whether cracking is more closely related to film design, substrate deformation, drying conditions, or aging.

12. Improvement Perspectives CoatureVietnam Can Offer

CoatureVietnam does not view cracking simply as a fractured film.
We organize it through the 4M perspective as a difference in expansion and contraction between substrate and film.

Material: Materials

• Does paint flexibility suit the substrate?
• Is the film too hard?
• Is the primer and topcoat combination appropriate?
• Can the resin system withstand outdoor service?
• Is the paint suited to wood or plastic?

Method: Methods

• Is film thickness appropriate?
• Is there excessive film build?
• Is setting time appropriate?
• Are drying conditions too rapid?
• Is wood moisture content checked?
• Is plastic-component flexing considered?

Machine: Equipment

• Does drying-oven temperature rise too rapidly?
• Is workpiece temperature excessive?
• Is temperature uniform?
• Does the drying equipment suit the substrate?

Man: People

• Do operators understand substrate movement?
• Do they mistakenly assume that a hard film is always better?
• Do they understand excessive-build risks?
• Can they record crack locations?

For Management, establish paint-selection criteria by substrate, film-thickness control, drying-condition standards, wood moisture-content control, judgments on whether plastic parts can be coated, and weather-resistance checks for outdoor applications.

Conclusion

Cracking is a defect in which the coating film fractures.
However, the cause is not necessarily confined to the film.

Substrates expand and contract with temperature, humidity, load, and service environment.
Cracks develop if the film cannot accommodate that movement.

A hard coating film is not always a good one.
Flexibility suited to the substrate, film thickness, drying conditions, and compatibility with the service environment are important.

Differences in expansion and contraction between substrate and film require particular consideration for wood, plastics, thin sheet metal, outdoor products, and components exposed to temperature changes.

Coating the Future with Science and Care
CoatureVietnam assesses cracking through the substrate, film, drying conditions, and service environment, as well as visible surface fractures, and helps develop coating specifications that reduce recurrence.