The basics of preventing cracks by considering substrate movement, coating-system layers, and film thickness

Introduction

Cracks sometimes develop in the coating film on wood.

The finish looked good immediately after application.
Fine cracks appeared some time after drying.
Cracks developed along the wood grain.
Only the surface has fine cracks.
Deep cracks extend down to the substrate.
Only certain types of wood crack, even though the same paint is used.

When these problems occur, the paint is often the first thing suspected.

“Could the paint be too weak?”
“Should we switch to a harder paint?”
“Is the topcoat insufficiently durable?”
“Would adding more curing agent make it stronger?”

However, paint alone does not cause cracking in wood coatings.

Unlike metal and plastic, wood contains moisture and expands and contracts with changes in humidity.
The wood itself continues to move after coating.

Cracks develop when the coating film cannot accommodate this movement.

This article examines cracking in wood coatings through wood moisture content, expansion and contraction, coating-film hardness, primer and topcoat combinations, film thickness, and drying conditions, as well as the paint itself.

1. Wood Continues to Move After Coating

There is one point to understand first when considering wood coatings.

Wood is a material that continues to move after coating.

Wood absorbs and releases moisture.
It absorbs moisture more readily in humid environments.
It loses moisture more readily in dry environments.

As a result, wood expands and contracts.

This movement differs considerably from that of materials such as metal and glass.

No matter how good the coating finish looks, movement in the wood underneath places stress on the film.
Cracks develop if the film cannot withstand that stress.

In wood coating, the ability to accommodate substrate movement is therefore important, as well as the strength of the coating film.

2. A Hard Coating Film Is Not Always Better

When considering coating quality, people sometimes assume that a harder film is better.

A hard film can certainly resist scratching.
Hardness may be important in applications that require abrasion resistance or surface hardness.

However, a harder film is not necessarily better for wood coatings.

Wood expands and contracts with humidity.
If the coating film is too hard, it cannot flexibly accommodate that movement.

Stress then builds up in the film and causes cracks.

The balance between coating-film hardness and flexibility is therefore important in wood coatings.

A strong coating film is not simply a hard one.
It is a film suited to the service environment and substrate movement.

3. Crack Patterns Help Indicate the Cause

When cracks develop in a wood coating, it is important to examine their pattern.

Cracks can be assessed in several ways.

• Are they fine cracks confined to the surface?
• Are they deep cracks extending to the substrate?
• Do they follow the wood grain?
• Do they have no particular direction?
• Do they form an alligator-like pattern?
• Are they concentrated in specific areas?
• Are they spread across the entire surface?

For example, cracks that follow the grain or fiber direction may involve expansion and contraction of the wood or cracking in the substrate itself.

On the other hand, fine cracks with no particular direction across the entire surface call for investigation of curing conditions, excessive film build, internal film stress, and the combination of primer and topcoat.

The direction, depth, and location of cracks provide clues for analyzing the cause.

Rather than recording only “cracking present,” document the type of cracking with photographs and notes.

4. High Moisture Content Can Cause Cracking and Warping

Moisture-content control is extremely important in wood coating.

Moisture content is the proportion of moisture contained in the wood.

If wood is coated while its moisture content is high, moisture loss after coating or renewed moisture absorption in service can cause the wood to move more readily.

This can result in the following defects.

• Cracking
• Warping
• Distortion
• Foaming
• Coating-film delamination
• Uneven gloss
• Poor drying
• Dimensional changes

Checking wood moisture content before coating is a basic requirement.

For industrial products, furniture, building materials, and interior components requiring precision, moisture content must be controlled within the specified range.

Moisture may remain inside wood even when it looks dry.
Do not judge its condition by feel alone; check it with a moisture meter or similar instrument.

5. Humidity Control Is Particularly Important in Vietnam

Vietnam’s hot, humid conditions have a major effect on wood coating.

Humidity rises during the rainy season.
Air-conditioning conditions in warehouses and factories also change the moisture condition of wood.
Wood stored outdoors or in partially enclosed outdoor areas absorbs moisture more readily.

Wood movement also depends on whether the coated product is used in an air-conditioned room or a humid environment.

Factories in Vietnam should pay attention to the following points.

• Wood storage location
• Increased humidity during the rainy season
• Moisture content before coating
• Drying and conditioning after coating
• Storage conditions before shipment
• Humidity during transport
• The customer’s service environment
• Temperature and humidity differences from air-conditioned environments

Quality control for wood coating must include storage conditions before and after application, as well as the coating process itself.

6. Poor Primer and Topcoat Combinations Increase the Risk of Cracking

Wood coating sometimes involves combining primer, intermediate coat, and topcoat to build the coating film.

Large differences in the properties of these layers can cause cracking.

For example, the following combinations require attention.

• A soft primer beneath an excessively hard topcoat
• A primer that has not dried sufficiently
• Large curing shrinkage in the topcoat
• Weak adhesion between primer and topcoat
• Large differences in hardness between coating layers
• An unsuitable recoating interval

A coating film should not be considered as a single layer alone.
The entire build-up from primer to topcoat forms one coating system.

One layer alone is hard.
One layer alone is soft.
The primer is insufficiently dried.
The topcoat shrinks strongly.

These conditions can generate internal stress in the film and lead to cracking.

In wood coating, examine the entire coating-system build-up, rather than individual paints alone.

7. Excessive Film Build Increases the Risk of Cracking

Applying wood coatings more thickly does not necessarily produce a better finish.

Greater thickness may improve the sense of film build and smoothness.
However, applying too much in one pass increases the risk of cracking.

Excessive film build commonly causes the following problems.

• The surface dries first
• Drying within the film is delayed
• Solvent remains inside the film
• Curing shrinkage increases
• Internal stress increases
• Drying becomes uneven
• Cracks develop later

Even when a certain film thickness is required, building it in several applications may be more stable than applying a thick layer all at once.

In wood coating, properly drying each stage while building the film is more important than simply increasing its thickness.

8. Too Much Curing Agent Can Also Cause Cracking

Two-component paints require the base and curing agent to be mixed in a specified ratio.

This mixing ratio is extremely important.

The following ideas sometimes arise on the shop floor.

“Would adding extra curing agent help it cure better?”
“Let’s add more curing agent to dry it faster.”
“A little extra should be fine.”

However, excessive addition of curing agent is dangerous.

Too much curing agent can make the film excessively hard or disrupt the reaction balance.
The film may then lack flexibility, fail to accommodate wood movement, and develop cracks.

Following the specified mixing ratio is a basic requirement for two-component paints.
Mixing by eye should be avoided.

In wood coating, the important point is to create a stable film using the specified mixing ratio, rather than trying to strengthen it by adding extra curing agent.

9. Watch for Insufficient Curing at Low Temperatures

Insufficient curing at low temperatures also requires attention in wood coating.

Paint may dry and cure more slowly at low ambient temperatures.
Moving to the next process before sufficient curing may leave unstable areas inside the film.

Applying a topcoat, force-drying, or exposing the product to environmental changes after shipment in that condition may cause later cracking, gloss changes, or poor adhesion.

Check the following points.

• Room temperature during application
• Wood surface temperature
• Paint temperature
• Drying time
• Recoating interval
• Curing-agent mixing ratio
• Standard conditions for low temperatures
• Conditioning time

Although Vietnam is associated with a hot, humid climate, temperatures in coating areas vary by region, season, and air-conditioning conditions.

Temperature and humidity control are also important in wood coating.

10. Air Inside the Wood Can Contribute to Foaming and Cracking

Wood is a porous material.
It contains small voids and air internally.

If force-drying begins immediately after application, air inside the wood can expand and cause bubbles or blistering in the coating film.

Foaming not only impairs appearance but also creates weak points in the film.
These may lead to cracking or delamination.

Moving directly into high-temperature drying after application is not always suitable for wood coatings.
A setting time may be necessary: a short period after application that allows the coating film and substrate to settle.

Check the following points.

• Type of wood
• Moisture content
• Porosity
• Setting time after application
• Forced-drying temperature
• Rate of temperature rise
• Locations where foaming occurs
• Whether the film is excessively thick

In wood coating, the timing of entry into drying affects quality, as well as the drying-oven temperature.

11. Investigation Procedure When Cracking Occurs

When cracks develop in a wood coating, checking in the following order helps organize the possible causes.

11-1. Check Crack Direction

Do the cracks follow the grain or fiber direction?
Are they spread over the surface with no particular direction?
Are they confined to specific areas?

Cracks along the fiber direction suggest wood movement or substrate cracking.

11-2. Check Crack Depth

Are the cracks confined to the surface?
Do they extend into the primer?
Do they extend to the bare wood?

Deep cracks may indicate a greater influence from substrate movement or the underlying layers.

11-3. Check Moisture Content

Was the moisture content appropriate before application?
Did the wood absorb moisture during storage?
Does moisture content differ between lots?

Moisture content is an important item to check in wood coating.

11-4. Check Film Thickness

Is the film excessively thick?
Was too much thickness applied at once?
Is film thickness at each process stage appropriate?

Excessive film build increases the risk of cracking.

11-5. Check the Coating-System Build-Up

Is the primer, intermediate coat, and topcoat combination appropriate?
Has each layer dried sufficiently?
Is the recoating interval appropriate?

Check the balance of the entire coating system.

11-6. Check Curing-Agent Mixing and Drying Conditions

Is curing agent being added by eye?
Is the specified mixing ratio followed?
Is insufficient curing occurring at low temperatures?

Mixing-ratio control is particularly important for two-component paints.

12. Points to Watch in Factories in Vietnam

Factories applying wood coatings in Vietnam should pay attention to the following points.

12-1. Wood Storage Conditions

Storing wood in humid areas increases its moisture content.
Storage conditions before application directly affect quality.

12-2. Moisture-Content Changes During the Rainy Season

Wood absorbs moisture more readily during the rainy season.
If defect rates differ between the rainy and dry seasons, moisture-content control needs to be reviewed.

12-3. Prioritizing Appearance Through Excessive Film Build

Applying a thick film to improve appearance may lead to cracking later.
Divide the work into stages to stabilize the process, rather than trying to complete the finish at once.

12-4. Variations in Curing-Agent Mixing Between Operators

Adding curing agent by eye causes variations in quality.
Measurement, records, and training are necessary.

12-5. Rushing the Drying Process

Reducing drying time too far to increase productivity can cause insufficient internal drying and foaming.
Necessary waiting periods are also part of the quality process in wood coating.

13. Questions to Use During Customer Visits

For Factory Managers

• Have you received complaints about cracking or warping in wood coatings?
• Does cracking appear before or after shipment?
• Do defects increase during the rainy season or periods of high humidity?
• Is the burden of touch-up and recoating substantial?

For Production Engineering

• Do you measure wood moisture content?
• Do you have a moisture-content standard?
• Is the primer, intermediate coat, and topcoat system defined?
• Do you control film thickness at each process stage?
• Is setting time standardized?

For Quality Assurance

• Do you record the direction and depth of cracks?
• Do you distinguish cracks along the grain from cracks with no particular direction?
• Do you link moisture content to the cracking rate?
• Do you check how the coating-system build-up and film thickness relate to defects?
• Do you check the service environment after shipment?

For Purchasing

• Do purchase specifications include wood moisture-content requirements?
• Do you check quality variations between wood lots?
• Do you check coating suitability when switching to less expensive wood?
• Do you confirm storage and transport conditions with suppliers?

For Coating Shop Supervisors

• Are you adding curing agent by eye?
• Are you applying too much paint at once?
• Do you check drying conditions at low temperatures or high humidity?
• Are you beginning force-drying immediately after application?
• When cracking occurs, do you check the wood’s condition as well as the paint?

14. How CoatureVietnam Can Help

CoatureVietnam can help improve cracking in wood coatings through substrate control, coating-system build-up, and drying conditions, as well as paint selection.

Specifically, we can provide the following support.

• Organizing information on cracking locations
• Classifying crack direction and depth
• Defining moisture-content control items
• Reviewing the coating-system build-up
• Defining film-thickness control points
• Creating curing-agent mixing SOPs
• Standardizing setting time
• Checking drying conditions
• Creating operator checklists
• Preparing improvement proposals for customers

In wood coating, it is important to improve the substrate and process together, as well as considering paint changes.

Conclusion

Cracking in wood coatings is not just a paint problem.

Wood contains moisture and expands and contracts with humidity.
The wood itself continues to move after coating.

Cracks develop when the coating film cannot accommodate that movement.

Check the following points in wood coating.

• Wood moisture content
• Storage conditions
• Expansion and contraction
• The primer, intermediate coat, and topcoat combination
• Coating-film hardness and flexibility
• Film thickness
• Whether the film is excessively thick
• Curing-agent mixing ratio
• Drying conditions
• Setting time

Simply switching to a harder paint may not solve cracking.
What is needed is coating-film design and process control suited to wood movement.

CoatureVietnam’s guiding principle, “Coating the Future with Science and Care,” means carefully breaking down these causes and translating them into conditions the shop floor can reproduce.

Science means examining wood moisture content, expansion and contraction, coating-system build-up, film thickness, and drying conditions separately.
Care means creating standards that operators can follow without uncertainty, reducing touch-up work and complaints.

Approach cracking in wood coatings through substrate movement, as well as examining the paint.
That is the first step toward stable wood-coating quality.