Fundamentals of spray, electrostatic, electrodeposition, powder, and waterborne coating

When a coating problem occurs, conversations on the shop floor often sound like this.

“Could the material be faulty?”
“Could the operator’s application technique be wrong?”
“Would changing the thinner solve it?”
“Could we simply extend drying time a little?”

Coating materials, thinners, operator skills, and drying conditions are, of course, important.
However, judging causes on those factors alone can overlook the real cause.

Changing the coating method changes the conditions to control, the defects likely to occur, and the approach to corrective action.

Air spray and electrostatic coating require different checks.
Defects appear differently in electrodeposition and powder coating.
Waterborne and solventborne coatings require different drying approaches.
Roller coating and dip coating require process controls quite different from spray coating.

To stabilize coating quality, it is important to understand not only which material is used, but also which application method is being used.

1. Coating problems cannot be judged from the material alone

When a defect occurs, the material may be suspected first.

Examples include sagging, roughness, particle defects, insufficient thickness, poor adhesion, uneven color, poor gloss, pinholes, and blistering.

However, these defects are not necessarily caused by the material itself alone.

Even with the same material, the finish changes greatly under conditions such as the following.

• Gun distance changes
• Air pressure changes
• Thinning ratio changes
• Application speed changes
• Workpiece shape changes
• Humidity changes
• Baking oven temperature distribution changes
• Fixture grounding deteriorates
• Rinsing conditions change
• Recovered powder condition changes

These conditions can substantially alter the finish.

Coating is not simply applying material.
It is a process linking materials, equipment, working conditions, the environment, drying, and inspection.

Solving coating problems therefore requires examining the whole process for each method, rather than the material alone.

2. Atomization, distance, and air pressure determine air spray quality

Air spray coating uses compressed air to atomize the material into fine droplets and spray it onto the workpiece.
It is a common method used in many factories.

In this method, operator-controlled conditions strongly affect quality.

The following items are particularly important.

• Coating viscosity
• Thinning ratio
• Air pressure
• Delivery rate
• Gun distance
• Gun speed
• Overlap width
• Application sequence
• Thinner evaporation rate

If the gun is too close, excessive material may deposit and cause sagging or a rough finish.
If it is too far away, droplets dry in flight, causing roughness or insufficient thickness.

Excessively high air pressure increases mist dispersion and may increase material losses or roughness.
Excessively low pressure causes insufficient atomization, a rough finish, or uneven thickness.

Standardizing working conditions is therefore extremely important in air spray coating, alongside material control.

3. Higher voltage is not always better in electrostatic coating

Electrostatic coating charges material particles and attracts them to a grounded workpiece.
It can improve transfer efficiency and reduce material losses.

However, simply assuming higher voltage is better is risky.

The following conditions matter in electrostatic coating.

• Applied voltage
• Coating electrical resistance
• Thinner type
• Workpiece grounding
• Gun distance
• Humidity in the coating environment
• Workpiece shape
• Presence of metallic pigments

Excessive voltage may concentrate material at edges or make recesses difficult to coat.
Unsuitable coating resistance may prevent a sufficient electrostatic effect.

Humidity can also influence electrostatic coating in hot, humid environments such as Vietnam.
If the finish changes between rainy and dry seasons, check electrostatic and environmental conditions as well as the material.

4. Bath control determines electrodeposition quality

Electrodeposition immerses the workpiece in a coating bath and uses electricity to form a film.
It is an important method for rust-protective primers on automotive and metal parts.

Operators do not apply the coating with a gun.
Instead, the condition of the bath strongly determines quality.

The main items to check are the following.

• pH
• Conductivity
• Solids content
• Bath temperature
• Voltage
• Current
• Energizing time
• Surface preparation condition
• Rinsing condition
• Electrical contact through fixtures
• Baking conditions

Electrodeposition must not be judged only by external thickness.
Check whether sufficient film forms inside, in gaps, at joints, and around welds.

Rust protection is not determined solely by visible appearance.
Insufficient internal or gap thickness may allow rust to start there.

Particle or dirt defects do not necessarily originate only in the electrodeposition bath.
Sources exist throughout the line, including preparation sludge, iron particles, insufficient rinsing, oven contamination, and contaminated conveyor hangers.

Electrodeposition requires an integrated view of the coating bath, preparation, rinsing, electrical supply, and baking.

5. Powder coating requires process control even with zero VOCs

Powder coating uses powdered material containing no solvent.
It readily limits VOC generation and is effective for improving the working environment and reducing fire risks.

However, environmental advantages do not make powder coating simple.

The following conditions determine quality.

• Powder particle size
• Powder charging properties
• Moisture absorption by the powder
• Gun type
• Voltage
• Current
• Workpiece grounding
• Booth airflow
• Recovered powder condition
• Baking oven temperature distribution
• Cooling conditions

Powder may have difficulty entering recesses.
This occurs because the electric field lines have difficulty entering recesses due to electrostatic behavior.
It is called the Faraday cage effect.

Excessive powder thickness, voltage, or current may also cause back ionization, leading to craters or an orange-peel appearance.

A thicker application is therefore not always better.
Application quantity, electrical conditions, and baking conditions must be controlled appropriately.

6. Drying control is difficult in waterborne coating

Waterborne coating uses water as the main medium instead of solvent.
It is effective in reducing VOCs and odors.

However, removing water from the film is extremely important.

Water evaporation is strongly affected by temperature and humidity.
In hot, humid environments such as Vietnam, poor drying is especially likely during the rainy season or on humid days.

Pay attention to the following items.

• Booth temperature
• Booth humidity
• Flash-off time
• Oven heating profile
• Residual water in the film
• Wastewater treatment
• Color matching
• Insulation control in waterborne electrostatic coating

The surface may dry first and make internal water difficult to remove.
Heating in this state may cause popping, pinholes, or blistering.

Water conducts electricity readily, so high voltage can leak through the material supply system in waterborne electrostatic coating.
Insulation and safety control must therefore cover the tank, hoses, supply equipment, and electrostatic gun.

Waterborne coating is effective for environmental objectives, but a material change alone is insufficient.
It must be treated as a process change covering booths, ovens, wastewater, safety, and color approval.

7. Select special coating methods to suit product shape

Industrial coating has many methods beyond spraying.

Examples include the following.

• Precoated metal
• Roller coating
• Curtain flow coating
• Dip coating
• Rotational coating
• Centrifugal coating
• Wipe coating

These are examples of alternative methods.

Depending on product shape or volume, these methods may offer advantages over spray coating.

Roller or curtain flow coating may suit flat materials.
Rotational coating may be effective for small parts.
Wipe coating principles may be useful for rods or wires.
Precoated metal is another option for metal sheet before fabrication.

Do not choose a method simply because it has always been used.

Select the best method by considering shape, production volume, thickness, appearance and rust-protection requirements, operator burden, and equipment investment.

8. Control items differ by coating method

The control items for each method can be organized as follows.

Even the same coating defect therefore requires a different approach to causes depending on the method.

9. Climate and working environment also matter in Vietnamese factories

Temperature, humidity, the rainy season, and working conditions affect coating quality in Vietnam.

The following processes are particularly susceptible.

• Charging behavior in electrostatic coating
• Drying of waterborne coatings
• Moisture absorption by powder materials
• Electrodeposition bath temperature
• Drying and baking oven temperature stability
• Operator health and safety

For example, high humidity makes water removal difficult in waterborne coatings.
Moisture absorption may change powder flow and charging properties.
Humidity and grounding affect electrostatic transfer efficiency.

Vietnamese factories therefore cannot simply apply the same assumptions about coating conditions as in Japan.
Controls must suit local climate, equipment condition, and working conditions.

10. Coating improvements must not depend solely on operator effort

Experienced operators’ skills may support coating quality.
This is an important strength.

However, stable quality should not rely solely on intuition or effort.

The following systems are needed.

• Application condition sheets
• Viscosity control sheets
• Film thickness measurement criteria
• Gun distance and air pressure standards
• Voltage and grounding check sheets
• Electrodeposition bath control sheets
• Recovered powder control sheets
• Baking oven temperature records
• Temperature and humidity records for waterborne coating
• Checking procedures for abnormalities

The operator is not necessarily at fault.
The underlying problem is often that conditions enabling consistent acceptable production have not been established.

This is precisely CoatureVietnam’s principle of “Coating the Future with Science and Care.”

Science means making coating conditions numerically visible and calmly isolating causes.
Care means reducing excessive dependence on operator workload or intuition and moving toward a process where everyone can consistently produce acceptable products.

11. Basic checks when selecting a coating method

Check the following items when considering a method.

1. Workpiece shape

Is it flat, box-shaped, complex, small, or rod-shaped?
The suitable method changes with shape.

2. Production volume

Is it low-volume production of many products or continuous high-volume production?
Automation and continuous processing offer greater benefits at high volumes.

3. Required thickness

Is a thin film sufficient, or is a thick film needed?
Powder or airless coating may be candidates when thick films are required.

4. Appearance requirements

Is a high-quality appearance required, or is functionality the priority?
Metallic or pearlescent effects, gloss, surface finish, and color difference requirements affect method selection.

5. Rust protection requirements

Where rust protection is important, consider preparation, electrodeposition, undercoats, and thickness control together.

6. Environmental requirements

Powder or waterborne coating may be candidates when VOCs, odors, or fire risks need to be reduced.
Each, however, presents different control challenges.

7. Drying and baking conditions

Check whether the workpiece tolerates heat and the oven has sufficient capacity.

8. Operator workload

The method changes working posture, odors, dust, heat, and cleaning burden.

9. Equipment investment

A method change may require booths, ovens, recovery systems, wastewater treatment, or safety equipment.

10. Total cost

Assess transfer efficiency, defect rate, recoating, disposal, power, wastewater, and labor, as well as material unit price.

12. Summary: understanding the method changes how problems are viewed

Solving coating problems requires understanding each method’s characteristics, rather than looking only at materials.

Air spray coating has its own approach.
Electrostatic coating has its own approach.
Electrodeposition has its own approach.
Powder coating has its own approach.
Waterborne coating has its own approach.
Special methods have their own approaches.

The same thickness defect has different causes depending on the method.
The same particle defect has different sources depending on the process.
The same drying defect requires different checks in solventborne and waterborne coating.

That is why the following questions matter in coating improvement.

Does the method suit the product shape, production volume, and quality requirements?Is the material truly the only cause of the defect?Have equipment, working, drying, and environmental conditions been checked?

CoatureVietnam reviews the coating process comprehensively, including methods, equipment, working conditions, surface preparation, drying, and inspection, as well as materials.

Coating the Future with Science and Care

This means understanding the process through numbers and systems and establishing conditions in which people on the shop floor can consistently produce acceptable products.

Related articles

• Air spray fundamentals—atomization, distance, and air pressure change film thickness
• Sagging, roughness, and rough finishes in spray coating are not just material problems
• Airless coating suits thick films, but nozzle and pressure control determine quality
• Higher voltage is not always better in electrostatic coating
• Coating resistance and thinner selection determine the electrostatic finish
• What is electrodeposition?—forming rust-protective films with electricity
• Bath control determines electrodeposition quality—understanding pH, conductivity, and solids content
• Where do dirt and particle defects arise in electrodeposition?
• Assess internal thickness and throwing power in electrodeposition
• What is powder coating?—process control is needed even with zero VOCs
• Differences between corona and tribo guns
• Why powder fails to enter recesses—the Faraday cage effect
• Thicker powder coatings are not always better—back ionization and recoating
• Baking ovens, cooling, and recovered powder control in powder coating
• Why is drying control difficult in waterborne coating?
• Pay attention to color matching and metamerism when switching to waterborne coating
• Waterborne electrostatic coating requires insulation and safety control
• What is precoated metal?—coating before fabrication
• What is a roller coater?—efficient coating of flat materials
• Dip, rotational, and curtain flow coating—mass production alternatives to spray coating
• Centrifugal and wipe coating—controlling excess material
• Coating method selection checklist—considering shape, production volume, and quality requirements