Preventing Defects Through Application Conditions, Drying and Curing, and Aging

When problems occur on a coating shop floor, paint is often the first thing suspected.

“Could the paint be defective?”
“Could it be a different lot?”
“Could the thinner be unsuitable?”

Paint or thinner can certainly be the cause.
However, actual coating problems do not arise from paint alone.

Application distance, air pressure, film thickness, thinning ratio, working speed, setting time, drying-oven temperature, workpiece temperature, humidity, compressed-air condition, substrate cleanliness, packaging materials, storage conditions, and service environment.

These conditions combine to produce defects such as cissing, dirt nibs, sagging, bubbles, blushing, metallic mottling, pinholes, wrinkling, after-tack, bleeding, yellowing, fading, and chalking.

Coating-process problems therefore need to be considered through the entire process, as well as the paint.

1. First Isolate Coating Defects by When They Occur

When analyzing a coating problem, the defect name is not the first thing to check.

The first point to check is when the defect occurred.

For example, even defects that look similarly whitish have different causes depending on when they occur.

Whitening immediately after application may involve high humidity or the thinner’s evaporation rate.
A whitish haze after drying may involve surface haze or contamination.
White powder after prolonged outdoor use suggests chalking, or deterioration of the coating-film surface.

Likewise, a defect described as holes requires different countermeasures depending on whether it is bubbles immediately after application, pinholes during drying, or gas released from the substrate.

Organizing coating defects by their timing as follows helps isolate the causes.

• Defects during application
• Defects during drying
• Defects during handling after drying
• Defects after packaging, storage, or transport
• Defects that appear after time in service

Checking in this order helps assess the entire process objectively rather than seeking the cause solely in the paint.

2. Examine Defects During Application Through Working Conditions and the Environment

Common defects during application include cissing, dirt nibs, dirt deposition, sagging, bubbles, rough finish, and metallic mottling.

These often originate in working conditions or the environment rather than the paint itself.

For example, sagging does not necessarily mean the paint is defective.
It arises when several conditions combine, such as insufficient gun distance, excessive output, excessive film thickness, low viscosity, or slow drying.

The same applies to bubbles.
Excessive film build, rapid drying, bubbles in the paint, moisture on the workpiece surface, and moisture in compressed air may be involved.

For metallic mottling, the arrangement of metallic pigments matters as well as the paint color.
The same paint can look different depending on gun condition, air pressure, application distance, film thickness, and setting time before the clearcoat.

Check the following items when examining defects during application.

• Is the thinning ratio appropriate?
• Is viscosity controlled?
• Is film thickness excessive?
• Is spray-gun distance appropriate?
• Is air pressure too high or too low?
• Is compressed air contaminated with moisture or oil?
• Is dust or oil present in the coating area?
• Are temperature and humidity within a suitable range for coating?

Coating work tends to rely heavily on operators’ experience.
However, stable quality requires making conditions visible and standardizing them, as well as drawing on experience.

3. Drying Is a Process That Develops Coating-Film Performance, Not Simply Waiting Time

Shop floors sometimes judge whether a film has dried by how its surface feels.
However, a film that feels dry to the touch is not necessarily ready to withstand assembly, packaging, or shipment.

Paint gradually changes into a coating film after application.
Solvent leaves, viscosity rises, the surface settles, and chemical reactions or bake curing proceed where required, ultimately developing film performance.

Drying is therefore more than simply waiting.
It is a quality-development process that builds coating-film hardness, adhesion, chemical resistance, and durability.

The drying-oven set temperature is not the only important factor.
What truly matters is the temperature reached by the workpiece itself and how many minutes it remains at that temperature.

Even when oven temperature meets the specified value, thick or large workpieces may not reach a sufficient actual temperature.
Thin workpieces, conversely, may be overbaked.

The following checks are important in bake drying.

• Is workpiece temperature measured rather than oven temperature alone?
• Is the required temperature maintained for the required duration?
• Is setting time appropriate?
• Is temperature distribution inside the drying oven uniform?
• Are solvent vapors properly exhausted?
• Is the oven air clean?
• Are differences in heating due to thickness, material, and coating color considered?

The drying oven is more than equipment located at the end of the coating line.
It is essential equipment for developing coating-film performance.

4. Insufficient Setting Time Causes Many Drying Defects

Setting time is the period after application during which the film is allowed to settle before entering a drying or baking oven.

During this time, the film surface settles, some solvent escapes, and leveling progresses.
If setting time is too short, rapid heating while solvent remains inside the film can cause pinholes, foaming, solvent popping, orange peel, or poor gloss.

Excessively long setting time can also cause problems.
Dirt may adhere, metallic pigment orientation may become disturbed, or the film surface condition may change.

Setting time is therefore not an arbitrary period of leaving the product standing.
It is a control condition for stabilizing the film.

Factories in Vietnam in particular may prioritize productivity and put products into the drying oven immediately after application.
However, rushing drying can increase defect rates and ultimately cause major losses through touch-up and recoating.

5. Recoating Without Checking the Substrate and Existing Film Is Risky

Repair coating and recoating require additional care compared with new coating.

The issue is that the type and condition of the existing film may be unknown.

An insufficiently cured existing film may be attacked by topcoat solvent, causing wrinkling or lifting.
Particular care is needed with deteriorated existing films, lacquer films, films with poor solvent resistance, and two-component films with insufficient curing agent.

Filler repairs, marking ink, tar, pitch, and undried primer can also cause bleeding or uneven color after topcoating.

Check the following for recoating or repair coating.

• Is the existing coating-film type known?
• Is the existing film free from deterioration?
• Can it withstand the topcoat solvent?
• Has the primer dried sufficiently?
• Has the filler dried completely?
• Do markings or dye-based inks remain?
• Is tar or pitch present?
• Does a trial application show bleeding or wrinkling?

Repair coating is more than applying another coat.
It requires the skill to assess the existing film.

6. Defects Can Also Occur During Packaging, Storage, and Transport After Drying

Quality is not permanently established when a coated product leaves the drying oven.

Packaging immediately after drying.
Stacking coating films against one another.
Placing vinyl sheets or protective films in close contact.
Transporting products in hot warehouses or containers.

These conditions can cause after-tack, blocking, packaging adhesion, pressure marks, or plasticizer migration.

After-tack is a phenomenon in which a film that appeared dry becomes tacky again later.
It involves retained slow-evaporating solvent, plasticizers in packaging materials, high-temperature storage, or softening of thermoplastic coating films.

For export products in particular, transport temperatures and sealed packaging create substantial risks.
Products shipped from Vietnam to Japan, ASEAN, or other regions require consideration of high container temperatures, long transport durations, and contact with packaging materials.

Check the following to assure coating quality.

• How many hours after drying are products packaged?
• Are packaging materials in close contact with the film?
• Are coated surfaces stacked against one another?
• Could storage or transport involve high temperatures?
• Do packaging materials contain plasticizers?
• Are pressure-contact or non-tack tests conducted?

Coating quality must be considered through packaging, storage, and transport, as well as within the coating line.

7. Initial Appearance Alone Cannot Assess Changes Over Time

Some defects appear over time even when there is no problem immediately after application.

Typical examples are bleeding, yellowing, fading, haze, and chalking.

Bleeding occurs when color constituents in the substrate or existing film migrate into the topcoat and change its color.
Yellowing occurs when coating-film constituents take on a yellow tint over time.
Fading is color loss caused by light, heat, oxygen, moisture, chemicals, or other factors.
Haze is a whitish clouding of the film surface that reduces gloss or transparency.
Chalking occurs when surface resin deteriorates and produces white powder.

These defects are difficult to detect through initial inspection alone.

Outdoor products in particular require checks on long-term color stability, gloss retention, and powdering, as well as initial color.

Vietnam has many factors that accelerate film deterioration, including strong sunlight, heat and humidity, rain, dust, and coastal salt.
For outdoor products, building materials, equipment covers, and visible components, aging must therefore be considered from the paint-selection stage.

8. Basic Checks for Preventing Coating Problems

To reduce coating-process problems, standardize routine checks as well as responding after defects occur.

At a minimum, check the following items.

• Does the paint meet the specification?
• Is the specified thinner being used?
• Is the thinning ratio appropriate?
• Is viscosity controlled?
• Is film thickness appropriate?
• Are spray-gun distance, air pressure, and output appropriate?
• Is compressed air free from moisture and oil?
• Are temperature and humidity in the coating area appropriate?
• Is the specified setting time followed?
• Is workpiece temperature in the drying oven measured?
• Are the primer and existing film checked?
• Is conditioning time before packaging sufficient?
• Has compatibility between packaging materials and the film been checked?
• Are fading and chalking risks in outdoor service considered?

These checks help narrow down causes quickly and prevent recurrence.

9. CoatureVietnam’s Approach to Coating-Process Improvement

CoatureVietnam believes that coating problems should not be dismissed as shop-floor mistakes or blamed solely on paint.

Coating defects burden the shop floor.
Recoating and inspection increase, delivery is delayed, and operators may feel responsible without understanding the cause.

That is why coating processes need science.

Science does not mean displaying complex theories.
It means organizing when, where, and under what conditions defects occur, and making their causes visible as process conditions.

Care means creating criteria that allow shop-floor staff to make decisions without uncertainty.
It means enabling operators to consistently produce good coating films without undue strain.

Paint, thinner, coating method, drying oven, booth, compressed air, packaging, storage, and service environment.
Connecting these factors in our thinking helps reduce coating problems.

CoatureVietnam organizes coating processes through the 4M perspective, as well as examining materials.

• Material: Paint, thinner, curing agent, substrate, packaging materials
• Method: Coating method, film thickness, drying conditions, inspection criteria
• Machine: Spray guns, booths, drying ovens, compressed-air equipment
• Man: Operator judgment, training, standard work

Where necessary, improvement also includes Management: control standards and SOPs.

Conclusion

Coating-process problems cannot be solved through paint alone.

Working conditions during application.
Drying and baking temperatures and times.
The substrate and existing film condition.
Packaging, storage, and transport conditions.
Changes over time in the service environment.

All of these affect coating-film quality.

Do not assume a cause from the defect name alone.
First check when the defect occurred.
Next organize the process conditions involved.
Then standardize them to prevent recurrence.

Coating the Future with Science and Care
This approach means improving coating processes in ways everyone can understand, reducing operators’ burden, and increasing quality and productivity rather than leaving everything to the shop floor.

CoatureVietnam organizes coating problems in Vietnam through paint, processes, equipment, and people, and develops improvement proposals that can be used in practice.