Consider product shape, production volume, and quality requirements

When coating problems occur, the shop floor often checks the following points.

Is the paint suitable?
Is the thinner suitable?
Is the operator’s application method correct?
Are drying conditions satisfactory?
Is surface preparation sufficient?

All of these are important.

However, there is another important perspective.

It is whether the application system itself suits the product.

Even with the same paint, air spray and electrostatic coating produce different film thickness distributions and transfer efficiencies.
Electrodeposition and powder coating have different approaches to rust prevention and process control.
Drying control is a major issue in waterborne coating.
Roller coaters and curtain flow suit flat materials, but may be unsuitable for complex shapes.
Dipping and rotational coating can be effective for small parts, but require attention to pooling and contact damage.

An application system should not be selected out of habit.
Nor should it be selected simply because the equipment is available.

The most rational system must be selected by considering product shape, production volume, quality requirements, the working environment, equipment conditions, and total costs.

1. Clarify the purpose before selecting an application system

There is a point to establish first before selecting an application system.

It is what you want the coating to achieve.

The purpose of coating differs between products.

Examples of its purposes include the following.

• Improve appearance
• Prevent rust
• Improve weather resistance
• Provide chemical resistance
• Improve abrasion resistance
• Provide electrical insulation
• Provide slip properties
• Enable identification by color
• Change the feel of the surface
• Improve the working environment
• Reduce VOCs
• Reduce paint losses
• Increase productivity

The optimum application system changes with the purpose.

Appearance-focused parts and rust-prevention-focused parts require different conditions to be considered.
High-mix, low-volume parts and high-volume continuous-production parts also suit different systems.
Where environmental requirements are a priority, waterborne and powder coating become candidates, but drying and baking equipment must be checked.

Application system selection should begin with defining the purpose, before choosing paint.

2. Check 1: product shape

Product shape is the first factor to examine when selecting an application system.

Product shape has a major influence on paint deposition, film thickness distribution, drying, and ease of application.

Items to check are as follows.

• Is it flat?
• Is it a box shape?
• Are there recesses?
• Are there holes?
• Are there inner surfaces?
• Is it tubular?
• Is it a small part?
• Is it a large part?
• Are there many curved surfaces?
• Are there many edges?
• Is pooling likely?
• Are air pockets likely?
• Is it easy to hold with fixtures?

Roller coaters or curtain flow may suit flat materials.
Dipping, rotation, or centrifugal coating may be candidates for small parts.
Spraying or robotic coating may be effective for complex shapes with demanding appearance requirements.
Electrodeposition is a candidate when rust prevention requires a film on inner surfaces as well.
Powder coating is a candidate for metal products prioritizing thick films and environmental requirements.

Selecting a system unsuitable for product shape prevents stable quality, regardless of operator effort.

3. Check 2: production volume and product variety

The next factors to examine are production volume and product variety.

Even with the same product shape, the suitable system changes with production volume.

Items to check are as follows.

• Is it high-mix, low-volume production?
• Is it mass production?
• Are batch sizes large?
• Are color changes frequent?
• Are product sizes consistent?
• Is the production takt time demanding?
• Will quantities fluctuate in the future?
• Is it suitable for automation?
• Can manual work accommodate production?

Versatile spray coating may suit high-mix, low-volume production with frequent color changes.
For large quantities of the same product, continuous systems such as automatic coating, robots, powder lines, roller coaters, and curtain flow can be considered.

For processing large quantities of small parts, dipping or rotation may be more rational than spraying individual items.

Installing large dedicated equipment for low production volumes prevents recovery of the investment.
Conversely, continued reliance on manual work at high volumes increases quality variation and operator workload.

Application systems must be selected according to production volume and product variety.

4. Check 3: required film thickness

Required film thickness is extremely important in application system selection.

Items to check are as follows.

• Is a thin film required?
• Is a thick film required?
• What film thickness variation is acceptable?
• Is coating of flat areas alone sufficient?
• Is thickness also required at edges and recesses?
• Is thickness required on inner surfaces?
• Is film thickness directly linked to performance?
• Is there a risk of defects from excessive thickness?

Airless or powder coating may be candidates when thick films are required.
Roller coaters or curtain flow may suit a uniform thin finish.
Electrodeposition needs consideration where internal rust prevention is important.

However, thicker is not always better.

Excessive thickness leads to sagging, drying defects, popping, pinholes, blisters, cracking, and increased costs.
In powder coating, excessive deposition can cause back ionization or a rough finish.

Both insufficient and excessive film thickness are problems.
The target should be the appropriate thickness for the product requirements.

5. Check 4: appearance requirements

Appearance requirements also have a major influence on application system selection.

Items to check are as follows.

• Is a high-quality appearance required?
• Are gloss requirements strict?
• Is color-difference control required?
• Is the finish metallic or pearlescent?
• Are surface texture requirements strict?
• What amount of inclusions or dirt is acceptable?
• Are repair marks acceptable?
• Are visible and concealed surfaces identified separately?
• Is a customer-approved sample available?

Demanding appearance requirements make spray conditions, drying conditions, booth cleanliness, film thickness control, and color matching important.

Metallic and pearlescent finishes require checking application direction, film thickness, drying conditions, and appearance under different light sources.

For small parts that prioritize function and have less demanding appearance requirements, systems such as dipping, rotation, or centrifugal coating may be rational.

Selecting a system prone to contact damage for a product with demanding appearance requirements causes problems.
Selecting an unnecessarily expensive system for a product with low appearance requirements increases process costs.

Appearance requirements must be checked without either excess or deficiency.

6. Check 5: rust-prevention and durability requirements

Rust prevention and durability can be important for metal products.

Items to check are as follows.

• Will it be used indoors?
• Will it be used outdoors?
• Is the environment highly humid?
• Will it be used near the coast?
• Will it be exposed to water?
• Will it contact chemicals?
• Are there requirements such as salt spray testing?
• Are edges or inner surfaces at risk of rust?
• Is a primer needed?
• Is electrodeposition or a rust-preventive primer needed?

Demanding rust-prevention requirements require checking surface preparation, film thickness, edge treatment, inner-surface thickness, and drying and baking conditions, as well as paint.

Electrodeposition may be effective for complex metal parts requiring internal rust prevention.
Airless or powder coating becomes a candidate where thick-film corrosion protection is needed.
For outdoor weather resistance, check paint type, thickness, baking conditions, and the combination with the substrate.

Rust-prevention quality cannot be judged by appearance alone.
It is important to check whether the required coating film is present in rust-prone areas.

7. Check 6: substrate material

The suitable application system changes with substrate material.

Materials to check include the following.

• Iron
• Galvanized steel sheet
• Aluminum
• Stainless steel
• Magnesium
• Plastic
• Wood
• Glass
• Paper
• Composites

For metals, surface preparation, rust prevention, grounding, and baking temperature are important.
For plastics, thermal deformation, solvent cracking, adhesion, and suitability for electrostatic coating are important.
For wood, moisture content, absorption, cracking, and dimensional changes are important.
For paper and film, absorption, drying, warping, and surface treatment are important.

Powder and baking coating require checking whether the substrate can withstand baking temperature.
Electrostatic coating also requires checking whether the substrate can be grounded.

Choosing an application system without checking the material may cause poor adhesion, deformation, fracture, cracking, or drying defects.

8. Check 7: drying and baking conditions

Drying and baking conditions must always be checked when selecting an application system.

Items to check are as follows.

• Is it air-dried?
• Is it force-dried?
• Is baking required?
• Can the material withstand heat?
• Is drying oven capacity sufficient?
• Can actual product temperature be checked?
• Will thick and thin parts be mixed?
• Is flash-off needed for waterborne coating?
• Are baking conditions suitable for powder coating?
• Can sufficient cooling time be provided?

Waterborne coating requires drying control that removes moisture appropriately.
In powder coating, depositing powder alone does not complete the film; melting and curing in a baking oven are required.
Underbaking leads to insufficient adhesion, hardness, and durability.
Overbaking leads to discoloration, gloss changes, and embrittlement.

Quality will not stabilize if the chosen application system cannot be accommodated by the drying and baking equipment.

9. Check 8: operator workload and safety

Operator workload and safety are also important in application system selection.

Items to check are as follows.

• Are working postures strenuous?
• Do operators need to lift heavy parts?
• Is prolonged spraying required?
• Is solvent odor strong?
• Is there dust exposure?
• Is high voltage involved?
• Is high-pressure coating equipment used?
• Is heat exposure around the baking oven high?
• Does cleaning involve hazards?
• Is the protective equipment appropriate?
• Are explosion protection and ventilation required?

Spray coating involves solvent mist, odor, and working-posture burdens.
Airless coating has safety risks from high-pressure injection.
High voltage and grounding control are important in electrostatic coating.
Powder coating requires attention to dust, static electricity, and baking oven heat.
Waterborne electrostatic coating requires insulation control because water and high voltage are combined.

Application systems should be selected for operator safety as well as quality.

This directly relates to the care in CoatureVietnam’s valued approach of bringing science and care to coating.

10. Check 9: environmental requirements

Environmental requirements are also important in application system selection.

Items to check are as follows.

• Do VOCs need to be reduced?
• Does odor need to be reduced?
• Does fire risk need to be reduced?
• Is wastewater treatment required?
• Is dust control required?
• How much waste paint is generated?
• How much waste thinner is generated?
• Can recovered powder be reused?
• Will the system save energy?
• Does the customer have environmental requirements?

Powder coating is effective for VOC reduction but requires control of dust, recovered powder, and baking ovens.
Waterborne coating is effective for VOC reduction but presents drying and wastewater treatment issues.
Electrostatic coating can potentially increase transfer efficiency and reduce paint losses.
Precoated metal may reduce the workload of in-house coating processes.

Environmental measures sometimes eliminate one problem while creating a different control issue.

Environmentally oriented coating must therefore consider equipment, exhaust, wastewater, waste, and operator safety as well as paint.

11. Check 10: equipment investment and total costs

Equipment investment and total costs must also be checked when selecting an application system.

Items to check are as follows.

• Initial equipment costs
• Coating booth costs
• Drying and baking oven costs
• Exhaust and wastewater equipment costs
• Fixture costs
• Maintenance costs
• Paint consumption
• Paint losses
• Recoating costs
• Defect rate
• Operating labor
• Cleaning labor
• Energy costs
• Waste treatment costs
• Risk of quality complaints

Inexpensive equipment does not necessarily make an inexpensive process.
Even expensive equipment may offer a total-cost advantage if it substantially reduces defect rates, paint losses, operating labor, and cleaning labor.

Judging by paint unit price alone is also risky.

Poor transfer efficiency, high defect rates, and frequent recoating increase total costs even with inexpensive paint.

Application system selection must consider the cost of the entire process, rather than the purchase price alone.

12. Basic comparison by system

The features of each application system can be organized as follows.

This table is not an absolute basis for judgment.
In practice, decisions combine product shape, paint, equipment, and quality requirements.

13. Practical application system selection checklist

Considering an application system in the following order makes decisions easier.

1. Product information

• Product name
• Substrate material
• Shape
• Size
• Weight
• Service environment
• Visible and concealed surfaces
• Rust-prone areas
• Areas prone to pooling
• Difficult-to-dry areas

2. Quality requirements

• Color
• Gloss
• Appearance criteria
• Film thickness
• Adhesion
• Rust prevention
• Weather resistance
• Chemical resistance
• Abrasion resistance
• Customer approval conditions

3. Production conditions

• Monthly production quantity
• Batch size
• Color-change frequency
• Takt time
• Automation requirements
• Potential for future production increases
• Number of operators
• Current defect rate

4. Equipment conditions

• Current application system
• Booth
• Drying oven
• Surface preparation
• Exhaust
• Wastewater
• Fixtures
• Handling and transport
• Inspection equipment
• Equipment replacement plans

5. Environment and safety

• VOC
• Odor
• Dust
• High voltage
• High-pressure equipment
• Fire risk
• Working posture
• Protective equipment
• Wastewater treatment
• Waste treatment

6. Costs

• Paint unit price
• Paint consumption
• Transfer efficiency
• Operating labor
• Defect rate
• Recoating
• Cleaning
• Energy
• Waste
• Equipment investment
• Maintenance

Organizing these 6 areas allows application systems to be compared based on facts rather than intuition.

14. Always perform trial coating before changing systems

Application system changes must not be decided by desk-based review alone.

Trial coating must always be performed on actual products or samples closely representing them.

Items to check in trial coating are as follows.

• Appearance
• Color
• Gloss
• Film thickness
• Film thickness distribution
• Edges
• Recesses
• Inner surfaces
• Pooling
• Sagging
• Drying characteristics
• Baking condition
• Adhesion
• Rust prevention
• Application properties
• Takt time
• Repairability
• Whether customer approval can be obtained

Advance verification is essential particularly for powder coating, electrodeposition, waterborne coating, roller coaters, and dipping, where the process principle differs substantially from the existing system.

A system change may be a larger change than a paint change.
Trial coating, mass-production trials, and customer approval should proceed in stages.

15. Common mistake: starting with equipment

A common mistake in application system selection is deciding on the basis of equipment first.

Examples include the following judgments.

• Spray because an existing booth is available
• Use powder coating because powder equipment is available
• Use electrodeposition because an electrodeposition line is available
• Adopt the system because inexpensive equipment can do it
• Use the same system because the product resembles an earlier product

Equipment constraints are important.
However, deciding solely on equipment may result in a system unsuitable for the product’s quality requirements.

The proper sequence is as follows.

1. Define product requirements
2. Check quality risks
3. Compare suitable application systems
4. Check whether existing equipment can accommodate them
5. Consider additional equipment, fixtures, and conditions needed
6. Verify through trial coating
7. Standardize as mass-production conditions

It is important to start with the product and quality requirements, rather than equipment.

16. Common mistake: trying to resolve everything through paint

Another mistake is trying to resolve coating problems through paint alone.

Paint selection is certainly important.
However, when the cause lies in the system and process conditions, changing paint does not resolve the underlying problem.

For example:

• Powder does not enter recesses
• Electrodeposited film thickness is insufficient on inner surfaces
• Popping occurs in waterborne coating
• Streaks occur with a roller coater
• Pooling occurs in dip coating
• Airless coating produces excessive thickness

These problems require checking the application system, equipment, fixtures, drying, and operating conditions, as well as paint.

Changing paint is one measure.
However, it is not the only measure.

Coating improvement requires reviewing Method, Machine, Man, and Management as well as Material.

17. CoatureVietnam’s 4M plus Management perspective

CoatureVietnam organizes application system reviews through the 4Ms plus Management.

Material: materials

• Paint
• Thinner
• Powder coating materials
• Surface preparation chemicals
• Substrate material
• Auxiliary materials

Method: methods

• Application system
• Film thickness conditions
• Thinning conditions
• Application procedure
• Drying conditions
• Inspection method

Machine: equipment

• Coating guns
• Booth
• Drying oven
• Electrodeposition tanks
• Powder recovery equipment
• Roller coater
• Fixtures
• Handling and transport

Man: people

• Operator skills
• Training
• Safety awareness
• Working posture
• Decision criteria
• Inspection practices

Management: control and management

• Condition sheets
• SOP
• Records
• Defect history
• Customer approval
• Change control
• Equipment maintenance
• Cost control

An application system cannot be judged by an equipment name alone.
The 4Ms plus Management must be considered together to determine whether the process will be stable.

18. How CoatureVietnam can help

CoatureVietnam does not select application systems solely from a paint-sales perspective.
We comprehensively review products, processes, quality, equipment, operators, safety, and costs.

We can provide the following support.

Diagnosing the current application system

We check whether the current application system suits product shape, quality requirements, and production volume.

Comparing application systems

We compare candidates such as spraying, electrostatic coating, electrodeposition, powder coating, waterborne coating, roller coating, and dipping.

Designing trial coating

We define trial conditions, film thickness measurement locations, appearance checks, drying conditions, and performance checks.

Reviewing equipment and fixtures

We check coating machines, booths, drying ovens, fixtures, transport, and surface preparation equipment.

Paint and thinner selection

We consider paints, thinners, powder materials, and surface preparation chemicals suited to the system.

Operating standardization

We develop coating condition sheets, inspection criteria, checklists, and abnormality-response SOPs.

Supporting customer approval

We define sample panels, color differences, thickness, performance tests, and approval conditions.

Improvement proposals

We propose improvements that combine stable quality, reduced operator workload, higher productivity, and environmental measures.

19. Summary: select application systems suited to the product and process

Spraying is not the only application system.

Various systems are available, including air spray, airless, electrostatic coating, electrodeposition, powder coating, waterborne coating, precoated metal, roller coaters, dipping, rotation, curtain flow, centrifugal coating, and wipe coating.

Each has strengths.
Each also has points requiring attention.

The important question is not which system is best in general.

Which system is most rational for this product?Which system will be stable for these quality requirements?Which system is safe, sustainable in daily operation, and easy to improve for this factory?

Select from these perspectives.

The basic factors to examine in application system selection are as follows.

• Product shape
• Production volume
• Substrate material
• Required film thickness
• Appearance requirements
• Rust-prevention requirements
• Drying and baking conditions
• Operator workload
• Safety
• Environmental requirements
• Equipment investment
• Total costs
• Customer approval

Coating the Future with Science and Care

Science means judging application systems through facts about products, quality, processes, and costs, rather than intuition or habit.
Care means selecting processes that allow operators to consistently produce acceptable products without excessive strain, and creating quality customers can use with confidence.

CoatureVietnam works with customers to design suitable coating processes encompassing application systems, equipment, surface preparation, drying, inspection, and operating standards, as well as paint.