Fisheyes, Dirt Nibs, and Dust Contamination Should Be Considered from the Environment and Surface Pretreatment, Not Just the Paint

Fisheyes, Dirt Nibs, and Dust Contamination Should Be Considered from the Environment and Surface Pretreatment, Not Just the Paint

Coating Surface Defects Vary Greatly Depending on Cleanliness Before Coating

Common coating defects found at manufacturing sites include fisheyes, dirt nibs, and dust contamination.

Immediately after coating, the paint may move away from both sides and form a circular shape.

Small dirt nibs may appear on the coating film surface.

After drying, raised particles resembling dust may remain on the surface.

The surface may become rough.

When these defects occur, people often first think:

“Is the paint the problem?”

Of course, foreign matter in the paint, insufficient filtration, or incompatible paint can also be causes.

However, in reality, fisheyes, dirt nibs, and dust contamination are not caused only by paint.

They can also be caused by many factors, including the workpiece surface, surface pretreatment, compressed air, air around the coating area, work clothes, jigs, drying ovens, and storage conditions.

In other words, these defects need to be viewed as a cleanliness-management issue across the entire coating process, not simply as a paint problem.

1. What Is a Fisheye?

A fisheye is a phenomenon in which paint does not wet and spread evenly over the workpiece surface, causing the coating film to move away from both sides and create a circular exposed area.

Small circular shapes may appear on the painted surface, making it look as though the paint is avoiding a particular area.

The essence of a fisheye is:

The paint cannot spread evenly over the surface.

Paint forms a uniform coating film by wetting and spreading over the workpiece surface.

However, if oil, water, silicone, wax, fingerprints, cleaning residue, or foreign matter is present on the surface, the paint may have difficulty wetting that area.

As a result, the paint is pushed away and a fisheye forms.

When observing a fisheye, we need to ask:

Is there a problem with the paint?

Or is there something on the surface before coating that prevents the paint from wetting properly?

Distinguishing between these two possibilities is very important.

2. Main Causes of Fisheyes

The cause of a fisheye is often found on the workpiece surface.

Typical causes include:

• Residual oil
• Insufficient degreasing
• Fingerprints or oil from hands
• Silicone-based contamination on the surface
• Mold release agent residue
• Wax or protective oil
• Water remaining on the surface
• Insufficient rinsing after surface pretreatment
• Insufficient drying
• Oil or water in compressed air
• Paint mist or contaminants in the working environment
• Contamination from wiping cloths or gloves

One particularly important point is that even when a surface looks clean, oil or silicone may still exist as an extremely thin layer.

In coating, not only visible contamination matters.

What matters is whether the paint can properly wet the surface.

Even a small amount of contamination on the surface can prevent the paint from spreading evenly.

Therefore, comprehensive checks are necessary, from degreasing, cleaning, and surface pretreatment to drying, glove management, and compressed-air management.

3. What Are Dirt Nibs and Dust Contamination?

Dirt nibs are defects in which small raised particles appear on the coating-film surface.

Dust contamination is a defect in which dust, fibers, paint residue, or other foreign matter adheres to the coating film and remains as contamination on the surface.

Both occur when foreign matter enters or adheres to the coating film.

Foreign matter may already exist in the paint.

But that is not the only possibility.

Foreign matter may also fall from the air during coating.

Fibers may fall from work clothes.

Old coating-film residue may fall from jigs or hangers.

Dust inside the drying oven may be carried up by hot air.

Outdoor air may also carry dust into the coating area.

Therefore, dirt nibs and dust contamination need to be considered from the perspective of foreign-matter control across the entire coating area.

4. Main Causes of Dirt Nibs and Dust Contamination

The causes can be broadly divided into four groups.

First: Foreign Matter from the Paint

Gelation in the paint, pigment agglomeration, paint residue, insufficient filtration, or contamination by old paint can be causes.

In such cases, check paint storage conditions, agitation, filtration, expiration date, and lot management.

Second: Foreign Matter from the Workpiece

Water droplets after surface pretreatment, insufficient drying, sanding dust, rust dust, machining dust, welding slag, plastic powder, wood dust, and similar materials can remain and appear as dirt nibs after coating.

The cleanliness of the workpiece before it enters the coating area is very important.

Third: Foreign Matter from the Coating Environment

Foreign matter can come from floors, walls, ceilings, spray booths, supply-air filters, exhaust systems, work clothes, air lines, jigs, and hangers.

In particular, if airflow around the spray booth becomes unstable, dust from the floor or outside can easily be carried into the coating area.

Fourth: Foreign Matter from the Drying Process

Paint residue, dust, tar-like deposits, peeling deposits on oven walls, or dust carried by hot air can be causes.

If the surface remains clean immediately after coating but the number of dirt nibs increases after drying, the inside of the drying oven should also be checked.

5. Fisheyes and Dirt Nibs May Look Similar, but Their Causes Are Different

Both fisheyes and dirt nibs appear on the coating-film surface.

However, their causes are very different.

A fisheye is a defect in which the paint cannot properly wet the surface.

The cause is often contamination on the surface, such as oil, silicone, water, or mold release agent.

By contrast, dirt nibs and dust contamination are caused by foreign matter entering or adhering to the coating film.

Possible causes include foreign matter in the paint, dust from the workpiece, dust in the environment, or contamination inside the drying oven.

If these two types of defects are confused, the corrective action may not be appropriate.

For example, simply strengthening paint filtration may not improve a fisheye problem.

Conversely, simply increasing deglazing may not solve a dirt-nib problem.

When observing a defect, first consider its shape:

• Has the paint moved away to both sides and formed a circular shape?
• Is there a small raised particle?
• Is there foreign matter at the center?
• Is the defect concentrated in a specific area or spread across the entire surface?
• Did it appear before drying?
• Or did it become apparent only after drying?

This observation is the first step in cause analysis.

6. Points That Require Special Attention at Vietnamese Manufacturing Sites

At coating factories in Vietnam, several conditions can easily lead to fisheyes, dirt nibs, and dust contamination.

The first is high temperature and humidity.

When humidity is high, water can remain on the workpiece surface.

Water can also enter compressed air.

Water can cause fisheyes, blushing/whitening, air bubbles, and adhesion defects.

The next factor is dust.

Dust from roads around the factory, machining processes, sanding, welding, or packaging areas can enter the coating area.

Particular attention is required when sanding or cutting operations are carried out near the coating area.

Outside air entering the coating area is also important.

If exhaust is increased while the supply-air volume is insufficient, the coating area can become negatively pressurized and draw dusty outside air through doors or gaps.

In this situation, no matter how thoroughly the spray booth is cleaned, dust will continue to enter from outside.

Compressed-air management is also important.

If the compressed-air dryer, condensate drainage, and oil mist separator are insufficient, water and oil in the air can enter the coating film and cause fisheyes or dirt nibs.

At Vietnamese factories, the focus should not be only on paint. The following should be considered together:

air, water, dust, supply/exhaust air, and compressed air.

7. Points to Check at the Manufacturing Site

When fisheyes, dirt nibs, or dust contamination occur, the following sequence can help systematize the causes.

1. Check the Defect Shape

• Has the paint moved away to both sides and formed a circular shape?
• Is there a raised particle?
• Is there foreign matter visible at the center?
• Does it appear across the entire surface or only in one area?
• Is it visible immediately after coating or only after drying?

2. Check the Workpiece Surface

• Is oil still present?
• Is degreasing sufficient?
• Is water still present after surface pretreatment?
• Is mold release agent or wax still present?
• Is sanding dust or machining dust present?
• Was the surface touched with bare hands?

3. Check the Paint and Thinner

• Has the paint been sufficiently agitated?
• Is there foreign matter or gel in the paint?
• Is the paint being filtered?
• Are there problems with the expiration date or storage conditions?
• Is the specified thinner being used?
• Is the paint viscosity appropriate?

4. Check the Compressed Air

• Is condensate drained regularly?
• Is the compressed-air dryer operating?
• Is an oil mist separator installed?
• Is there water or oil in the air lines?
• Are the air lines contaminated?

5. Check the Coating Environment

• Is the spray booth being cleaned?
• Are supply-air filters replaced periodically?
• Is the balance between supply air and exhaust air appropriate?
• Is outside air carrying dust into the area?
• Are work clothes and gloves generating fibers?
• Is old coating residue present on jigs or hangers?

6. Check the Drying Oven

• Is there dust or paint residue inside the oven?
• Is dust being carried upward by hot air?
• Are deposits on the oven walls peeling off?
• Does the number of dirt nibs increase after drying?

In other words, paint, workpieces, air, equipment, operators, and the drying oven should all be checked in sequence.

8. Recurrence Prevention Requires Standardization, Not Just Cleaning

When fisheyes or dirt nibs occur, temporarily strengthening cleaning is necessary.

However, that alone cannot prevent recurrence.

What is important is to standardize cleaning and inspection.

For example, standards should be established for:

• Degreasing before coating
• Prohibition of touching surfaces with bare hands
• Glove replacement
• Paint filtration
• Checking condensate in compressed air
• Spray booth cleaning frequency
• Replacement criteria for supply-air filters
• Cleaning standards for jigs and hangers
• Drying oven cleaning standards
• Recording rules when defects occur

The objective is not simply to react when defects occur.

The goal is to create a daily operating condition in which defects are less likely to occur.

This is coating quality management at the process level.

9. Improvement Perspective Proposed by CoatureVietnam

At CoatureVietnam, fisheyes, dirt nibs, and dust contamination are not viewed simply as paint defects.

We systematize their causes from the 4M perspective.

Material

• Is there foreign matter in the paint?
• Is the thinner appropriate?
• Are the degreaser and surface pretreatment chemicals appropriate?
• Is there contamination from wiping cloths, gloves, or packaging materials?

Method

• Is the degreasing method appropriate?
• Is the cleaning process before coating clearly defined?
• Is the paint filtered?
• Is bare-hand contact prevented?
• Could dust be introduced through the workflow route?

Machine

• Are the supply and exhaust air conditions of the spray booth appropriate?
• Is the compressed air clean?
• Is the inside of the drying oven properly cleaned?
• Can old coating residue fall from jigs or hangers?

Man

• Do operators understand the difference between fisheyes and dirt nibs?
• Can defects be recorded when they occur?
• Are cleaning and inspection standards being followed?
• Can the operator decide whether to stop the process when a defect is detected?

When necessary, the Management perspective can also be added.

In other words, cleaning frequency, inspection criteria, responsible personnel, defect records, and recurrence-prevention rules need to be established.

Summary

Fisheyes, dirt nibs, and dust contamination are easily regarded as paint defects because they appear on the coating-film surface.

However, many factors are actually involved, including the surface condition before coating, surface pretreatment, degreasing, compressed air, the spray booth environment, work clothes, jigs, and cleanliness inside the drying oven.

A fisheye is a defect in which the paint cannot properly wet the surface.

Dirt nibs and dust contamination are defects caused by foreign matter entering or adhering to the coating film.

Distinguishing between these two types of defects helps determine more appropriate corrective actions.

The focus should not be only on the paint. Cleanliness needs to be checked from before coating through drying.

Temporary cleaning is not enough; it needs to be standardized as a daily management activity.

At the same time, standards should be established so that operators at the manufacturing site can make their own judgements.

These are the basic principles for reducing fisheyes, dirt nibs, and dust contamination.

Science and Care in Coating.

CoatureVietnam supports defect reduction at manufacturing sites and reduces the burden on workers by looking at paint, processes, equipment, and the working environment as a whole.