A basic perspective for avoiding conclusions based on the defect name alone

When a defect occurs in a coating shop, the first thing people often focus on is the name of the defect.

Cratering.
Dirt inclusions.
Sagging.
Bubbles.
Blushing.
Pinholes.
Wrinkling.
Bleeding.
Yellowing.
Chalking.

Identifying a defect correctly is, of course, important.
However, it is risky to decide the cause immediately from the defect name alone.

This is because even defects that look similar can have very different causes depending on when they occurred.

For example, suppose a whitish defect appears.
If it turned white immediately after coating, high humidity or the thinner’s evaporation rate may be involved.
If it became white and hazy after drying, hazing or surface contamination may be responsible.
If white powder appeared after outdoor use, chalking, meaning deterioration of the coating film surface, may be involved.

Even defects all described as white can have different causes and countermeasures.

This is why, when examining a coating defect, the first question must be:

When did this defect occur?

1. Classify defect occurrence into five stages

Coating defects can broadly be organized into five occurrence stages.

The first category comprises defects that occur during coating.
Examples include cratering, dirt inclusions, sagging, bubbles, poor surface texture, and metallic mottling.
In these cases, check not only the coating material but also gun distance, air pressure, film thickness, viscosity, thinner, coating area cleanliness, and compressed air condition.

The second category comprises defects that occur during drying.
Examples include pinholes, foaming, solvent popping, wrinkling, and lifting.
In these cases, check flash-off time, drying temperature, baking temperature, workpiece temperature, primer drying, solvent release, and oven air conditions.

The third category comprises defects that occur during handling after drying.
Examples include scratches, pressure marks, blocking, coating films sticking together, and a return of tackiness.
In these cases, check the time allowed after drying, the time before packing, stacking, packaging materials, and storage temperature.

The fourth category comprises defects that occur after packing, storage, or transport.
Examples include packaging material sticking to the film, plasticizer migration, tackiness after transport, and compression marks.
In these cases, examine not only the coating film itself but also packaging materials, sealed packaging conditions, temperatures during transport, and the environment inside the container.

The fifth category comprises defects that occur after a period of use.
Examples include bleeding, yellowing, fading, hazing, chalking, and cracking.
In these cases, examine not only the initial coating conditions but also the use environment, ultraviolet light, heat, moisture, chemicals, substrate movement, and coating film deterioration over time.

Organizing defects by occurrence timing in this way makes it possible to investigate along the process rather than assume a cause based on intuition.

2. For defects appearing during coating, examine working conditions and the environment

Defects visible immediately during coating are readily affected by working conditions and the environment.

For example, sagging is not necessarily a coating material problem alone.
Working conditions may be involved, such as excessive film thickness, low viscosity, a short gun distance, a high material flow rate, or an excessively slow-drying thinner.

The same applies to cratering.
In addition to coating compatibility, check oil, moisture, silicone, and dust on the workpiece surface, insufficient surface preparation, and oil in compressed air.

For dirt inclusions or contamination, the cause may be not only foreign matter in the coating material but also air in the coating area, booth cleaning, work clothing, jigs, dust inside the drying oven, or incoming outside air.

The following questions help when examining defects during coating.

• Is the defect visible immediately after coating?
• Does it occur frequently with a particular operator, gun, or booth?
• Is the film too thick?
• Are viscosity and thinning ratio controlled?
• Is air pressure too high or too low?
• Is moisture or oil entering the compressed air?
• Is coating area cleanliness maintained consistently?
• Does the defect rate change when temperature or humidity changes?

Although defects during coating are easy to detect on the shop floor, several causes often overlap.
This makes recording working conditions and checking reproducibility important.

3. For defects appearing during drying, examine what is happening inside the coating film

For defects that occur during drying, examining only the coating film surface may not reveal the cause.

Inside the film, solvent escapes, resin solidifies, and curing reactions proceed where required.
If internal solvent or gas cannot escape effectively during this process, pinholes or foaming occur.

Topcoat solvents can also attack the primer or existing film, causing wrinkling or lifting.
If acidic gases or combustion gases enter the drying oven, defects such as gas checking may also occur.

The following questions help when examining defects during drying.

• Are parts placed in the drying oven immediately after coating?
• Is flash-off time sufficient?
• Is the film too thick?
• Is workpiece temperature measured in addition to the drying oven setpoint?
• Has the primer dried sufficiently?
• Have the existing coating type and its solvent resistance been checked?
• Is drying oven exhaust sufficient?
• Are solvent vapors or contaminating gases accumulating inside the oven?

For drying defects, the idea that faster drying is always better can be risky.
Rapid drying can instead trap solvent or gas inside the coating film.

Drying is not simply waiting time.
It is the process that completes the coating film.

4. For defects appearing after drying, examine handling conditions

Even when coated products look satisfactory as they leave the drying oven, defects may appear afterward.

Typical examples include a return of tackiness, blocking, packaging material sticking to the film, pressure marks, and scratches.

A return of tackiness means that a film believed to have dried becomes tacky again later.
Possible causes include slow-drying solvent left inside the film, insufficient drying, softening of a thermoplastic film, and plasticizer migration from packaging materials.

Particular attention is needed because the condition immediately after drying differs from the condition required for packing.

The film can be touched with a finger without a problem.
The product can be carried.
Products can be stacked.
The product can be wrapped in close contact with packaging.
The product can withstand transport at high temperatures.

These are not all the same condition.

The following questions help when examining defects after drying.

• How many hours after drying are products packed?
• Are coating films stacked against one another?
• Are packaging materials in close contact with the film?
• Are vinyl sheets or protective films used?
• Will products be stored or transported in hot environments?
• Does tackiness or sticking occur after packing or transport?
• Are compression or non-tackiness tests performed?

Coating quality does not end at the drying oven exit.
Quality must be considered through packing, storage, and transport.

5. For defects appearing later, examine the use environment and deterioration over time

A coating may look good immediately after application yet develop defects days, weeks, or months later.

For example, bleeding is a defect in which color components from the underlying material or existing coating migrate into the topcoat and change its color.
Yellowing is a phenomenon in which coating film components develop a yellow tint over time.
Fading is loss of color caused by light, heat, oxygen, moisture, and other factors.
Hazing is a phenomenon in which the film surface becomes whitish and hazy, reducing gloss and transparency.
Chalking occurs when resin at the film surface deteriorates and produces white powder.

These defects may not be found in initial inspection.

For products used outdoors in particular, it is necessary to check not only initial color but also color retention, gloss retention, and resistance to chalking.

The following questions help when examining defects that appear later.

• How long after coating did the defect occur?
• Did it occur during indoor storage or after outdoor use?
• Is the product exposed to direct sunlight, high temperatures, high humidity, or chemicals?
• Is yellowing noticeable on white or pale colors?
• Is the color prone to fading, such as red or purple?
• Does white powder come off when touched by hand?
• Are color difference and gloss checked over time?

Coating quality cannot be judged solely from appearance immediately after application.
It is important to examine how consistently the coating performs in the environment where the product is used.

6. An initial diagnosis table by occurrence timing

Organizing coating defects as follows makes it easier to distinguish their causes.

This table can be used for initial diagnosis when a defect is found on the shop floor.
The important point is to examine occurrence timing and process conditions together rather than judge from the defect name alone.

7. The perspective CoatureVietnam emphasizes

CoatureVietnam believes that coating defect investigations should not end simply with blaming someone’s mistake or the coating material.

Defects always occur under particular conditions.

When did it occur?
Where did it occur?
In which lot?
With which operator?
With which coating material and thinner?
At what humidity?
Under which drying conditions?
Did it occur after packing?
Did it occur after use?

Organizing this information makes defects understandable in terms of process conditions rather than impressions.

This is what CoatureVietnam means by science.

Then translate the findings into criteria, checklists, and SOPs so shop-floor operators can make decisions without uncertainty.
This is what CoatureVietnam means by care.

Conclusion

When examining a coating defect, the first step is not deciding the cause from the defect name alone.

First, check when it occurred.

Was it during coating?
During drying?
After drying?
After packing or transport?
After a period of use?

This distinction alone clarifies which processes should be checked.

Coating problems cannot be solved through coating materials alone.
Considering coating materials, thinners, working conditions, drying conditions, underlying surfaces, packaging, and the use environment together is what enables prevention of recurrence.

Coating the Future with Science and Care
CoatureVietnam organizes coating defects from the perspective of the entire process and turns that understanding into practical improvement proposals for the shop floor.