When the finish is inconsistent in electrostatic coating, attention on the shop floor often turns to the coating equipment’s voltage or gun settings.

“Is the voltage too low?”
“Is there something wrong with the gun?”
“Is the operator’s application technique at fault?”

Voltage, gun distance, grounding, and operating conditions are, of course, important.
However, electrostatic coating has one more important item to check.

That is coating material resistivity.

Put simply, coating material resistivity describes how readily the material conducts electricity.
Electrostatic coating charges coating particles and attracts them to the workpiece, so the material’s electrical properties have a major effect on the finish.

This resistivity depends not only on the coating material itself, but also on the thinner used.

In electrostatic coating, therefore, thinner is more than a material for diluting the coating.
It is an important process condition that influences electrostatic coating quality.

1. Coating particles must acquire an appropriate charge in electrostatic coating

Electrostatic coating electrically charges coating particles and attracts them to a grounded workpiece.
The electrostatic effect will not be consistent unless the particles acquire an appropriate charge.

An inconsistent electrostatic effect can cause the following problems.

• Poor deposition of coating material on the workpiece
• No improvement in transfer efficiency
• Inconsistent film thickness
• Differences in film thickness between edges and flat surfaces
• Difficulty getting coating material into recesses
• A rough finish
• Greater coating material loss
• Higher safety risks

Electrostatic coating cannot be achieved by coating equipment alone.
It becomes stable only when the coating material, thinner, voltage, grounding, gun distance, and workpiece geometry are all suitable.

Among these factors, coating material resistivity is an important condition for assessing whether a material is suitable for electrostatic coating.

2. What is coating material resistivity?

Coating material resistivity is an indicator of how readily a coating material conducts electricity.

A material with low resistivity conducts electricity readily.
A material with high resistivity does not conduct electricity readily.

In electrostatic coating, this resistivity must be within an appropriate range.

If it is outside the appropriate range, the coating material may not charge properly, electrical charge may leak away, or the electrostatic effect may weaken.
As a result, the coating material will no longer deposit consistently on the workpiece.

It is important to understand that this is not simply a matter of “higher resistivity is better” or “lower resistivity is better.”

The appropriate range depends on the coating equipment, type of coating material, application method, and material supply system.
Check the conditions recommended by the coating material and equipment manufacturers, and control resistivity to suit shop-floor conditions.

3. Excessively low resistivity makes electrical leakage more likely

If coating material resistivity is too low, the material conducts electricity readily.
This can allow high voltage to leak into the coating material supply system.

In this state, it can be difficult to charge the coating particles sufficiently, and the electrostatic effect may weaken.

Potential problems include the following.

• No improvement in transfer efficiency
• Insufficient film thickness
• Greater coating material loss
• Unstable voltage
• Electrical instability at the gun tip or in the supply system
• Higher safety risks

Waterborne coatings and some other low-resistivity materials conduct electricity readily, making electrical isolation and safety measures particularly important.

4. Excessively high resistivity can also make charging unstable

Conversely, excessively high coating material resistivity also requires attention.

If resistivity is too high, coating particles may be difficult to charge as intended.
The electrostatic effect may then be insufficient, resulting in inconsistent transfer efficiency and film thickness distribution.

Depending on the combination of atomization, gun distance, and voltage settings, this can also lead to a rough or gritty finish.

In electrostatic coating, therefore, resistivity must not be judged simply as “high” or “low.”
It must be checked against the appropriate range for the application method.

5. Thinner changes coating material resistivity

Thinner selection is particularly important in electrostatic coating.

Thinner is used to dilute coating material and adjust its viscosity.
In electrostatic coating, however, it does more than that.

Different thinners can change coating material resistivity.
This affects the electrostatic effect, transfer efficiency, film thickness, appearance, and safety.

For example, the following judgments are sometimes made on the shop floor.

“It is a similar thinner, so we can use it.”
“Switching to a cheaper thinner should be fine.”
“The drying rate is similar, so it should be a suitable substitute.”
“Let’s dilute it with the thinner we have in stock.”

Even in conventional spray coating, these judgments require caution.
In electrostatic coating, they need even more careful verification.

Similar drying rates do not necessarily mean identical electrical properties.

Thinners for electrostatic coating must be selected with consideration for coating material resistivity and charging behavior, as well as drying characteristics.

6. Use caution when substituting a general-purpose thinner

Cost, availability, or inventory constraints may tempt a factory to use a general-purpose thinner instead of the specified one.

However, substituting a general-purpose thinner requires caution in electrostatic coating.

The specified thinner and a general-purpose thinner may differ in the following properties.

• Drying rate
• Solvency
• Coating material resistivity
• Charging behavior
• Leveling
• Sagging tendency
• Surface finish
• Safety
• Compatibility with coating equipment

A change in coating material resistivity can change the electrostatic effect itself.
The finish can therefore change even at the same voltage, gun distance, and application settings.

Before substituting a general-purpose thinner, always conduct an application trial and check film thickness, appearance, transfer efficiency, and safety.

7. The thinner’s drying rate also affects the finish

In electrostatic coating, the thinner’s drying rate is important as well as its electrical properties.

If drying is too fast, coating particles start drying before they reach the workpiece or before they can level sufficiently on its surface.
This can result in grittiness, a rough finish, or insufficient gloss.

If drying is too slow, sagging, pooling, inadequate drying, and dirt contamination become more likely.

Electrostatic coating efficiently attracts coating material to the workpiece, and under some conditions it can build up excessively in particular areas.
Using a slow-drying thinner in such circumstances can lead to sagging or defects caused by excessive film thickness.

Thinner selection requires simultaneous consideration of these two points.

1. Whether it produces resistivity suitable for electrostatic coating
2. Whether its drying rate suits the shop-floor environment and film thickness

Both are important.

8. Metallic coatings require additional care

Additional care is needed when using metallic coating materials for electrostatic coating.

Metallic coatings may contain metal pigments such as aluminum pigments.
These can affect the material’s electrical properties and pigment orientation during application.

Metallic coating can present the following problems.

• Uneven color
• Uneven metallic appearance
• Differences in appearance caused by film thickness variation
• Electrical instability
• Poor pigment orientation
• Color differences under different lighting angles
• Differences in appearance between edges and flat surfaces

Electrostatic coating attracts coating material to the workpiece electrically, so changes in film thickness distribution also affect the metallic appearance and perceived color.

For metallic coatings, resistivity, thinner, voltage, gun distance, application direction, and film thickness must be controlled together.

9. Vietnam’s climate makes thinner selection even more important

In Vietnam, coating environments can change readily because of high temperatures, high humidity, the rainy season, and differences in ventilation between factories.

Even with the same coating material and thinner, a change in environment can change the finish.

The following factors are particularly susceptible to environmental changes.

• Drying rate
• Grittiness
• Sagging
• Gloss
• Electrostatic effect
• Transfer efficiency
• Dirt contamination
• Effects of moisture

High temperatures make thinner evaporate more quickly.
High humidity can destabilize film formation and electrostatic conditions.
Strong booth exhaust can cause coating particles to dry in flight, making a gritty finish more likely.

In a Vietnamese factory, simply selecting thinner as one would in Japan is therefore insufficient.
Selection must be checked against local temperature and humidity, the booth environment, drying equipment, and operating conditions.

10. Thinner-related checks for electrostatic coating

When selecting or changing a thinner for electrostatic coating, check the following items.

1. Is it the thinner specified by the coating material manufacturer?

First, identify the thinner specified by the coating material manufacturer.
A thinner outside the specification must always be verified by testing.

2. Is it suitable for electrostatic coating?

The criteria used for a conventional spray-coating thinner are insufficient for electrostatic coating.
Check its effect on coating material resistivity.

3. Does coating material resistivity fall within the appropriate range?

Check that resistivity is within the appropriate range after the thinner has been added.

4. Does the drying rate suit the shop-floor environment?

Check that drying is neither too fast nor too slow for the temperature, humidity, booth air velocity, and film thickness.

5. Are film thickness and appearance consistent?

In an application trial, check film thickness, gloss, surface texture, grittiness, sagging, and uneven color.

6. Are there any safety concerns?

Check flammability, vapors, ventilation, explosion protection, and operators’ personal protective equipment.

7. Is supply reliable?

It is also important to ensure a supply arrangement that supports continued use in mass production.

11. Problems commonly encountered after changing thinner

Changing thinner can produce the following changes.

If defects appear after a thinner change, check whether electrostatic conditions have changed, as well as whether the drying characteristics are unsuitable.

12. Purchasing decisions must consider more than thinner unit price

Because thinner is a consumable, purchasing departments sometimes emphasize its unit price.
Cost control is, of course, important.

However, choosing thinner solely by unit price can increase the overall cost of electrostatic coating.

For example, changing to a cheaper thinner may result in:

• Lower transfer efficiency
• Higher coating material consumption
• Inconsistent film thickness
• A higher defect rate
• More recoating
• More booth contamination
• Longer working time
• A higher risk of customer complaints

These are all possible consequences.

In such cases, the thinner’s unit price falls, but the cost of the overall process rises.

When selecting thinner for electrostatic coating, consider the total cost represented by the following items.

• Thinner unit price
• Coating material consumption
• Transfer efficiency
• Defect rate
• Recoating labor
• Cleaning labor
• Safety management costs
• Risk of quality complaints

A cheap thinner is not necessarily economical overall.

13. A practical shop-floor checking sequence

When the finish is inconsistent in electrostatic coating, checking in the following order helps narrow down the cause.

1. Establish when the defects started

Was it after changing thinner?
After a change in coating material batch?
Since the start of the rainy season?
After equipment inspection?
After a change of operator?

Identify what changed.

2. Check the coating material and thinner combination

Is the thinner the specified product?
Is it a substitute?
Are different thinners being mixed?
Is the dilution ratio as specified in the standard?

3. Check coating material resistivity

Where possible, check the actual resistivity of the diluted coating material.

4. Check voltage and grounding

Check voltage, gun distance, and grounding condition as well as resistivity.

5. Check film thickness and appearance

Measure film thickness on flat surfaces, edges, recesses, and the back of the workpiece.
Determine whether thickness differences are related to the appearance defects.

6. Check drying conditions and the booth environment

Check whether the thinner’s drying characteristics suit the shop-floor temperature and humidity and the booth air velocity.

The effects of a thinner change must therefore be investigated by checking the coating material, electrical conditions, drying, and equipment together.

14. How CoatureVietnam can help

CoatureVietnam can help address electrostatic coating issues from the following perspectives.

Checking coating material and thinner suitability

We check whether the coating material and thinner combination is suitable for electrostatic coating.

Evaluating substitute thinners

Even when local sourcing or cost reduction is required, we evaluate substitutes while checking their electrostatic characteristics, drying behavior, and effects on quality.

Organizing application conditions

We organize voltage, gun distance, grounding, film thickness, and drying conditions so that they can be managed in a process conditions sheet.

Isolating the causes of defects

We investigate uneven film thickness, grittiness, sagging, uneven color, and poor deposition from the perspectives of coating materials, thinners, equipment, and operating conditions.

Operator training

We train operators to understand thinner as an important material affecting electrostatic coating quality, as well as a material used for dilution.

15. Conclusion: Thinner is a quality condition in electrostatic coating

In electrostatic coating, coating material resistivity and thinner selection determine the finish alongside voltage and gun settings.

Thinner is more than a material for dilution.
It is an important process condition affecting resistivity, charging behavior, drying rate, leveling, and safety.

Check the following points in electrostatic coating.

• Is coating material resistivity appropriate?
• Is the thinner suitable for electrostatic coating?
• Has the specified thinner been replaced?
• Does the drying rate suit the shop-floor environment?
• Is transfer efficiency stable?
• Are film thickness and appearance consistent?
• Are grounding and voltage conditions appropriate?

Thinner for electrostatic coating must not be selected on price alone.
Decisions must consider coating material resistivity, the finish, safety, and total cost.

Coating the Future with Science and Care

Science means checking resistivity and drying characteristics and managing them as process conditions.
Care means creating an environment in which operators can consistently produce good parts without being troubled by unstable material conditions.

CoatureVietnam checks coating materials, thinners, equipment, and operating conditions together in electrostatic coating, and supports improvements suited to the shop floor.

Related articles

• In electrostatic coating, simply increasing the voltage is not the answer
• Air spray coating basics: Atomization, distance, and air pressure change film thickness
• Sagging, grittiness, and a rough finish in spray coating are not caused by the coating material alone
• Waterborne electrostatic coating requires electrical isolation and safety management
• Coating method selection checklist: Consider product geometry, production volume, and quality requirements