Stabilizing wood coating quality through moisture, expansion and contraction, and bubbling

Introduction

Wood coating may experience the following problems.

Cracks appear after coating.
Bubbles appear after drying.
Warping or distortion develops after shipment.
The coating film blisters.
Gloss or appearance is unstable.
The same coating material produces different finishes on different wood lots.

When these problems occur, the coating material or drying conditions are often suspected.

Material selection and drying conditions are, of course, important.
However, wood coating has another important item to check first.

That item is the wood’s moisture content.

Moisture content indicates how much moisture is contained in the wood.
Wood absorbs and releases moisture.
It therefore expands and contracts with its environment even after coating.

Wood coating quality cannot be stabilized through the coating material alone.
The wood’s moisture condition, storage environment, drying after coating, and service environment must all be considered.

This article explains why moisture content control matters in wood coating, from the perspectives of bubbling, cracking, warping, and drying conditions.

1. Wood is a material that contains moisture

Wood differs greatly from metal and glass.

Metal generally does not contain moisture internally.
Glass is likewise not a material into which moisture enters and causes expansion and contraction.

Wood is different.

It contains moisture internally.
It also absorbs or releases moisture according to the surrounding humidity.

In humid environments, wood readily absorbs moisture.
In dry environments, moisture leaves the wood.

This movement of moisture causes wood to expand or contract.

Wood therefore does not become completely stationary after coating.
It continues to move with its environment.

Understanding this premise is extremely important in wood coating.

2. What is moisture content?

Moisture content is an indicator of the amount of moisture contained in wood.

Simply put, it is a number expressing how much water the wood contains.

High moisture content means much moisture remains inside the wood.
Low moisture content means the wood is relatively dry.

An unsuitable moisture content before coating makes various problems more likely afterward.

For example, if wood is coated with excessively high moisture content, internal moisture may leave afterward and cause shrinkage.
This puts force on the film and may cause cracking or peeling.

Expansion of internal moisture or air during drying can also cause bubbling or blistering.

Checking the wood’s moisture condition before application is fundamental to wood coating.

3. What problems can high moisture content cause?

Coating wood with high moisture content may cause the following problems.

• Cracking
• Warping
• Distortion
• Bubbling
• Blistering
• Film peeling
• Poor drying
• Uneven gloss
• Blushing
• Dimensional changes
• Appearance changes after shipment

Particular care is needed because problems may not be visible immediately after coating.

The finish looks good immediately after coating.
Inspection finds no problem.
However, the wood’s moisture changes during storage, transport, or use at the customer’s site, and defects appear later.

Such cases do occur.

Wood coating must consider moisture changes after application, rather than judging a product acceptable solely from its immediate appearance.

4. Wood expansion and contraction apply force to the film

Wood expands when it absorbs moisture and contracts when moisture leaves.

This expansion and contraction apply force to the coating film.

Problems are less likely if the film can accommodate the wood’s movement.
However, an excessively hard or thick film, or a poor balance between undercoat and topcoat, may prevent this.

Cracking then occurs.

The important point is that cracking does not necessarily mean the film is weak.

An excessively hard film can also crack.
It cannot accommodate wood movement, so stress becomes concentrated in the film.

Producing a hard film alone must not be the objective in wood coating.
The film must be designed to accommodate wood expansion and contraction.

5. Moisture and air inside wood can cause bubbling

Wood is porous.
It contains fine internal voids holding moisture and air.

If forced drying begins immediately after coating, the air and moisture inside the wood are heated.
Air or water vapor may then be pushed outward and cause bubbles or blisters in the film.

This is one cause of bubbling in wood coating.

Bubbling causes appearance defects.
It also creates weak points in the film that may later lead to peeling or cracking.

When bubbling occurs, check the following items rather than suspecting only the material.

• Wood moisture content
• Wood species
• Porosity
• Coating film thickness
• Flash-off time after application
• Forced-drying temperature
• Heating rate
• Timing of entry into the drying oven

It is risky to assume that products should enter the oven immediately after application.
In wood coating, a flash-off period before drying may be important.

6. Flash-off time stabilizes quality

Flash-off time is a defined standing period after application, before forced drying or the next process.

It allows the condition of the film surface and wood interior to settle.

Insufficient flash-off time in wood coating may cause the following defects.

• Bubbling
• Blistering
• Uneven drying
• Uneven gloss
• Sagging
• Solvent remaining inside the film
• Cracking that appears later

To improve productivity, it may be tempting to send products into the oven immediately after coating.
However, rushing can increase defects in wood coating.

Flash-off time is not simply waiting time.
It is a necessary process for stabilizing quality.

The following items should be standardized on site.

• Time between application and oven entry
• Temperature and humidity conditions
• Film condition
• Film thickness
• Wood moisture content
• Presence or absence of bubbling
• Relationship with defect occurrence

7. Wood storage conditions change moisture content

Moisture content is not determined only at purchase.
It changes with storage conditions.

For example, wood readily absorbs moisture under the following storage conditions.

• A humid warehouse
• A location where rain enters
• Partly outdoor storage
• A location exposed to moisture from the floor
• A warehouse without air conditioning
• Long storage during the rainy season
• Moisture trapped inside packaging

Wood condition can change during storage before coating.

Even if moisture content is appropriate on receipt, moisture absorbed during storage may change its condition by the time of coating.

Checking moisture content before coating is therefore important, as well as checking it on receipt.

Wood storage locations, clearance above the floor, ventilation, air conditioning, and rainy-season measures are also part of quality control.

8. Plan for the rainy season and high humidity in Vietnam

Vietnam’s hot, humid environment has a major influence on wood coating.

Humidity is particularly high during the rainy season.
If warehouse humidity control is insufficient, wood readily absorbs moisture.

Conditions also differ between regions.

The north, south, and central regions have different climates and humidity changes.
Near the sea, moisture and salt influences must also be considered.
Wood condition also differs between air-conditioned and non-air-conditioned factories.

The following points require attention when coating wood in Vietnamese factories.

• Rising moisture content during the rainy season
• Warehouse humidity
• Wood storage duration
• Dryness before application
• Conditioning environment after application
• Storage before shipment
• Humidity changes during transport
• The customer’s service environment

Wood coating processes cannot be designed without regard to climate.
Moisture content must be controlled with Vietnam’s hot, humid environment in mind.

9. Warping and distortion may emerge after coating

Unstable wood moisture content may cause warping or distortion after coating.

Warping means bending of board-shaped wood.
Distortion means dimensions or shape no longer remain as designed.

Even wood that looks satisfactory before coating may change shape as drying or changes in the service environment alter its internal moisture balance.

Take particular care when coating only one side or using different film systems on the front and back.
Uneven moisture movement may make warping more likely.

Check the following items.

• Front and back coating specifications
• Wood thickness
• Grain direction
• Moisture content
• Drying conditions
• Placement during storage
• Service environment after shipment
• One-sided or two-sided coating

Dimensional stability must be assessed as quality alongside appearance in wood coating.

10. Do not rely on impressions to control moisture content

Wood dryness is sometimes judged subjectively on the shop floor.

“It looks dry.”
“It feels fine.”
“We coated it in this condition before.”
“The weather is good today, so it should be fine.”

However, internal moisture cannot be assessed by appearance alone.

The surface may be dry while moisture remains inside.
Drying behavior also changes with species and thickness.

It is therefore preferable to measure moisture content whenever possible.

Check the following points.

• Is a moisture meter used?
• Are measurement locations defined?
• Is measurement frequency defined?
• Are acceptance criteria available?
• Are criteria defined for each wood species?
• Are results recorded?
• Can moisture records be reviewed when defects occur?

Wood coating requires numerical control rather than subjective impressions.

11. Consider moisture control together with the coating process

Moisture control is not limited to the period before application.
It must be linked to the entire coating process.

For example, even wood with appropriate moisture content may bubble if high-temperature drying starts immediately after coating.
Conversely, insufficient conditioning after coating may expose the film to moisture changes before it stabilizes.

Check the following items together with moisture content.

• Moisture content before application
• Substrate temperature before application
• Humidity during application
• Absorption of the undercoat into the wood
• Film thickness
• Flash-off time
• Drying temperature
• Drying time
• Heating rate
• Conditioning after application
• Storage environment before shipment

Moisture content, film thickness, and drying conditions should be considered as an integrated set of process conditions, rather than controlled separately.

12. Excessively thick application is risky even with appropriate moisture content

Even with appropriate moisture content, applying too much material at once increases the risk of cracking or bubbling.

A thick application may dry first at the surface and trap solvent or moisture inside.
It may also increase internal stress and make later cracking more likely.

In wood coating, applying several coats is effective even when a specified total thickness is required.

The following 3 points are important.

1. Control moisture content
2. Avoid excessively thick application
3. Allow sufficient drying at each stage

Together, these 3 points make wood coating quality easier to stabilize.

13. Basic items to check for moisture control

When controlling moisture content for wood coating, check the following items.

Wood

• Wood species
• Board thickness
• Grain direction
• Seasoned or unseasoned wood
• Laminated timber or solid wood
• Plywood or MDF
• Wood lot
• Substrate supplier

Storage

• Moisture content on receipt
• Storage humidity
• Clearance above the floor
• Rainwater influence
• Storage duration
• Packaging condition
• Presence or absence of air conditioning
• Rainy-season measures

Checks before application

• Moisture content before application
• Measurement locations
• Measurement frequency
• Acceptance criteria
• Surface temperature
• Visual appearance
• Presence of warping or cracks

Coating process

• Undercoat absorption
• Film thickness
• Number of coats
• Flash-off time
• Drying temperature
• Drying time
• Heating rate
• Conditioning time

Shipment and service environment

• Storage environment before shipment
• Humidity during transport
• Location of use
• Indoor or outdoor use
• Whether the environment is air-conditioned
• Whether humidity is high

14. Questions for customer visits

For factory managers

• Are there complaints about cracking, bubbling, or warping in wood coatings?
• Do defects increase during the rainy season?
• Is there much rework or recoating after application?
• Is humidity controlled in the wood storage area?

For production engineering personnel

• Do you measure moisture content before coating?
• Are reference limits defined for moisture content?
• Do you use separate criteria for each species?
• Is flash-off time after application defined?
• Are oven temperature and time suited to the wood?

For quality assurance personnel

• Do you link moisture content to defect rates?
• Do you record cracking, bubbling, and warping separately?
• Can you trace the wood lot when defects occur?
• Do you check the service environment after shipment?
• Do you analyze the relationship between storage humidity and defects?

For purchasing personnel

• Are wood moisture conditions included in purchasing specifications?
• Do suppliers provide information on dryness?
• Do you check coatability when switching to cheaper wood?
• Do you share storage and transport conditions with suppliers?

For coating supervisors

• Are you judging dryness by appearance alone?
• Do you use a moisture meter?
• Are products entering high-temperature drying immediately after application?
• Is application excessively thick?
• When bubbling occurs, do you check wood moisture as well as the coating material?

15. How CoatureVietnam can help

CoatureVietnam can support wood coating moisture control through substrate management, storage, drying, and process standardization, as well as material proposals.

Specific support includes the following.

• Organizing causes of wood coating problems
• Designing moisture content checks
• Checking wood storage conditions
• Classifying cracking, bubbling, and warping
• Standardizing flash-off time
• Reviewing drying conditions
• Identifying film thickness control points
• Preparing operator checklists
• Preparing improvement proposals for customers

Wood coating quality cannot be stabilized by reviewing materials alone.
The wood’s moisture condition and process conditions must be controlled together.

Summary

Moisture control matters because wood moves as its moisture changes.

Wood expands and contracts with humidity even after coating.
Coating at unstable moisture content makes cracking, bubbling, warping, distortion, and peeling more likely.

Check the following in wood coating.

• Moisture content before application
• Wood storage conditions
• Rainy-season and high-humidity influences
• Flash-off time
• Drying temperature
• Drying time
• Film thickness
• Whether application is excessively thick
• Service environment after shipment

Many wood coating problems cannot be solved simply by changing the material.
Review substrate moisture and organize the entire process.

CoatureVietnam’s principle of “Coating the Future with Science and Care” means understanding these substrate characteristics carefully and converting that understanding into controls reproducible on site.

Science means separating and considering moisture content, expansion and contraction, bubbling, and drying conditions.
Care means providing criteria operators can check without relying on impressions, reducing rework and complaints.

Wood coating begins with moisture control before application.
Controlling moisture content is the first step toward stable wood coating quality.