Small surface holes are a signal from inside the coating film

Small holes sometimes appear in the coating film after application.
Parts of the film may also swell or rise like bubbles.

These defects are called pinholes, foaming, solvent popping, bubbles, or similar terms.

Small holes appear on the surface.
Fine holes increase after baking.
The film looks as though bubbles have emerged from inside it.
Only thickly coated areas swell.
Holes concentrate at corners or in recesses.
Defects appear after the product enters the drying oven.

When these phenomena occur, coating material quality or oven temperature is often suspected.
Coating materials and drying conditions can, of course, be involved.

However, the essential issue in pinholes and foaming is not the surface holes themselves.

The essential issue is that solvent, air, moisture, or gas inside the film could not fully escape.

The small holes visible on the surface are therefore the result of phenomena inside the film.

1. What are pinholes?

Pinholes are defects consisting of small holes in the film surface, as though it had been punctured with a needle.

Because the holes are small, they may be difficult to see from a distance.
However, they may become visible when the lighting angle changes or be detected during inspection.

Pinholes occur when solvent or gas trying to escape from inside the film creates small passages through its surface.

If gas escapes while the film is still soft, the holes may close.
However, if the surface dries first, the paths left by escaping gas remain as holes.

In other words, pinholes are a defect caused by an imbalance between the timing of gas release from inside the film and the timing of surface solidification.

2. What is foaming?

Foaming is a phenomenon in which bubbles form inside the film, making it swell or creating bubble-like surface defects.

Foaming may appear larger than pinholes.
The film surface may swell, or bubbles may burst and leave holes.

The basic cause of foaming is also gas inside the film.

Possible sources of this gas include the following.

• Solvent left in the film
• Air entrained in the coating material
• Moisture on the workpiece surface
• Moisture inside the substrate
• Gas released from the substrate
• Solvent left inside primer or filler
• Vapor generated by rapid heating

These substances expand during drying or baking and try to escape.
However, if the film surface has already solidified, the gas is trapped and causes foaming.

3. Excessively thick application is a major cause of pinholes and foaming

Film thickness is one of the most important items to check for pinholes and foaming.

As the film becomes thicker, internal solvent and air have more difficulty escaping.

Solvent near the surface escapes relatively quickly.
Solvent deeper inside, however, takes longer to escape.

When a thickly coated product enters the drying oven, internal solvent is heated and evaporates rapidly.
The resulting gas tries to escape, causing pinholes or foaming.

Particular care is needed under the following conditions.

• A thick film is applied in one pass.
• Too much coating material is applied to obtain hiding power.
• Coating material collects at corners or in recesses.
• Overlapping areas become thick.
• The total primer and topcoat film thickness is high.
• Only filled or repaired areas are excessively thick.

Ensuring the required film thickness is important in coating.
However, applying more thickness than necessary does not improve quality.

Excessive thickness causes many defects, including inadequate drying, pinholes, foaming, a return of tackiness, sagging, and wrinkling.

4. Insufficient flash-off traps solvent inside

Flash-off time also has a major influence on pinholes and foaming.

Flash-off time is the period allowed for the film to settle after coating and before entering a drying or baking oven.

During this time, the film surface levels and some internal solvent escapes naturally.

If flash-off time is insufficient, the film is heated while substantial solvent remains inside.
The solvent then evaporates rapidly in the oven and tries to escape as gas.

If the film surface has begun to solidify at that point, the gas cannot escape cleanly.
This produces pinholes or foaming.

In baking or forced drying in particular, film temperature starts rising as soon as the product enters the oven.
Rapid heating without sufficient flash-off makes internal solvent move suddenly and encourages defects.

Flash-off time is not simply production waiting time.
It is an important process for allowing internal solvent to escape safely.

5. Rapid drying may solidify only the surface first

Faster drying helps improve productivity.
However, rapid drying can cause pinholes and foaming.

If drying is too rapid, only the film surface dries first.
Once the surface solidifies, internal solvent and gas have more difficulty escaping.

Gas generated inside afterward has nowhere to go and breaks through the surface, forming holes or bubbles.

This phenomenon is more likely under the following conditions.

• Too much fast-drying thinner is used.
• The drying oven entrance temperature is high.
• High-temperature drying starts immediately after coating.
• Hot air strikes the product too strongly.
• Thick film
• The workpiece temperature rises suddenly.
• Coating conditions cause only the surface to dry rapidly.

Faster drying is not necessarily better.
What matters is allowing solvent and gas to escape smoothly from inside to outside.

6. Pay attention to gas released from underlying layers or the substrate

Solvent in the coating material is not the only cause of pinholes and foaming.
Gas released from underlying layers or the substrate can also cause them.

For example, the following substrates and areas require care.

• Castings
• Die castings
• Zinc plating
• Porous materials
• Wood
• Molded plastic products
• Filler repairs
• Areas where the primer has not dried
• Substrates that have absorbed moisture

Castings and die castings may contain microscopic internal voids.
When heated, air or gas inside these voids may expand and push up the film.

In wood or porous materials, moisture may be heated into vapor and cause film swelling.

In molded plastic products, residual molding constituents, moisture, or mold release agents may have an effect.

When examining pinholes and foaming, it is therefore necessary to check the workpiece material and underlying surface condition as well as the coating material.

7. Insufficient primer or filler drying can also be a cause

A defect that appears to be pinholes or foaming in the topcoat may originate in a lower layer.

The primer has not dried sufficiently.
Solvent remains inside the filler.
The surfacer has been applied too thickly.
Drying is delayed only in repaired areas.
Solvent or moisture remains in the existing film.

If topcoating is performed in these conditions, solvent or gas emerges from the lower layers during drying.
It pushes up the topcoat film and causes pinholes or foaming.

In this case, changing the topcoat material does not address the underlying cause.

The primer and filler condition must be checked.

• Has the primer dried sufficiently?
• Has the filler dried all the way through?
• Has filler been applied too thickly?
• Are defects concentrated in repaired areas?
• Have the type and condition of the existing film been checked?
• Is a trial application performed before topcoating?

In repair coating, the drying condition of hidden lower layers is particularly important.

8. Check moisture and oil in compressed air

In spray coating, compressed air condition also affects pinholes and foaming.

Moisture in compressed air may enter the film together with the coating material.
Once inside, it may turn into vapor during drying or baking and cause bubbles or pinholes.

Oil contamination also causes cratering, dirt inclusions, or poor adhesion.

Moisture control in compressed air is particularly important during Vietnam’s rainy season.

The following points should be checked.

• Is the air dryer working correctly?
• Is condensate drained regularly?
• Has moisture accumulated inside the air piping?
• Is moisture removed immediately before the spray gun?
• Is the oil mist separator working?
• Does the defect rate rise during the rainy season?

Air used for coating is an important condition affecting film quality.

9. Distinguish causes by occurrence timing

Examining when pinholes and foaming occur makes it easier to distinguish their causes.

If bubbles are visible immediately after coating, check coating material agitation, spray air, the application method, viscosity, and moisture on the workpiece surface.

If holes or bubbles appear after entering the drying oven, check insufficient flash-off, rapid heating, excessive application thickness, oven entrance temperature, workpiece temperature, and solvent in lower layers.

If defects occur only in specific areas after baking, check gas inside the substrate, porosity in castings or die castings, filled areas, film thickness in recesses, and primer drying.

If defects are discovered after shipment, also check a return of tackiness, insufficient internal drying, pressure during packing, and high-temperature transport.

It is important to organize when the defect occurred, where it occurred, and which areas are most affected rather than judge by the defect name alone.

10. Points requiring particular attention in Vietnamese factories

Vietnamese factories have several conditions that encourage pinholes and foaming.

First is the hot and humid environment.
High humidity encourages moisture on workpiece surfaces, in compressed air, and inside substrates.
This moisture may turn into vapor during drying and cause defects.

Next is excessively thick application.
If a thick film is applied in one pass to obtain appearance or hiding power, internal solvent has difficulty escaping.

Insufficient flash-off caused by prioritizing productivity is also important.
Putting products into the drying oven immediately after coating causes internal solvent to evaporate suddenly and encourages foaming.

Substrate control is also important.
In die castings, castings, wood, molded plastic products, and similar materials, internal moisture or gas may have an effect.

Vietnamese factories need to check humidity, substrates, film thickness, drying ovens, and compressed air together with coating conditions.

11. Shop-floor checkpoints

When pinholes or foaming occur, check the following items.

Defect condition

• Are the holes small or large?
• Is the film swollen like bubbles?
• Are the areas around the holes raised?
• Does the defect occur everywhere or only in certain areas?
• Is it concentrated in thick-film areas?
• Is it concentrated in recesses or at corners?
• Does it increase after baking?

Coating conditions

• Is the film too thick?
• Is the coating applied too thickly in one pass?
• Is coating viscosity appropriate?
• Is air entrained during agitation?
• Is air pressure too high?
• Is the gun distance too short?
• Is the application speed appropriate?

Thinner and drying conditions

• Is the specified thinner being used?
• Is drying too fast?
• Is flash-off time sufficient?
• Is the drying oven entrance temperature too high?
• Is the initial drying temperature rise too rapid?
• Is workpiece temperature checked?
• Are solvent vapors exhausted sufficiently?

Underlying surface and substrate conditions

• Is moisture present on the workpiece surface?
• Is moisture or gas present inside the substrate?
• Does the defect occur in castings or die castings?
• Has the primer dried sufficiently?
• Has the filler dried all the way through?
• Does the defect occur on existing film or in repaired areas?

Compressed air conditions

• Is the air dryer functioning normally?
• Is condensate drained?
• Does the air contain moisture or oil?
• Do defects increase during the rainy season?

Checking these items in order makes it easier to narrow down the causes of pinholes and foaming.

12. Improvement perspectives CoatureVietnam can propose

CoatureVietnam does not view pinholes and foaming only as surface holes.
It examines them from a 4M perspective as the movement of solvent and gas inside the film.

Material: Materials

• Are the coating material’s drying properties appropriate?
• Is the specified thinner being used?
• Does the thinner drying rate suit the shop floor?
• Is solvent or moisture left in the primer, filler, or existing film?
• Does the substrate readily contain gas or moisture?

Method: Methods

• Is the film thickness appropriate?
• Is the coating applied too thickly?
• Is flash-off time sufficient?
• Is the timing of entry into the drying oven appropriate?
• Is primer and filler drying checked?
• Is a trial application performed to check whether defects occur?

Machine: Equipment

• Is the drying oven entrance temperature too high?
• Is workpiece temperature appropriate?
• Is exhaust sufficient?
• Are solvent vapors accumulating?
• Is the compressed air equipment functioning normally?
• Are the air dryer and filters working?

Man: Operators

• Do operators understand the risks of excessive application thickness?
• Do they follow the required flash-off time?
• Do they record when defects occur?
• Can they check underlying surfaces and the drying oven as well as the coating material?

From a Management perspective, it is important to establish film thickness standards, flash-off time standards, drying oven loading rules, workpiece temperature control, compressed air inspections, and SOPs for checking primer and filler drying.

Conclusion

Pinholes and foaming are defects that appear as small holes or bubbles on the film surface.

However, the essential issue is inside the film rather than at its surface.

Pinholes or foaming occur when solvent, air, moisture, or gas from the substrate cannot fully escape during drying or baking.

Possible causes include excessively thick application, insufficient flash-off, rapid drying, fast-drying thinner, insufficient primer or filler drying, moisture or gas inside the substrate, and moisture in compressed air.

Countermeasures must go beyond simply filling the holes.
Film thickness, thinner, flash-off time, the drying oven, workpiece temperature, underlying layers, substrates, and compressed air need to be reviewed comprehensively.

Coating the Future with Science and Care
CoatureVietnam treats pinholes and foaming as phenomena inside the coating film and supports the establishment of process conditions that prevent recurrence on the shop floor.