A Thicker Coating Film Does Not Necessarily Mean Better Quality

Common defects on coating shop floors include sagging and bubbles.

Paint flows downward in streaks.
Small bubbles remain on the coating surface.
Holes or raised areas are visible after drying.
The film runs at corners or on vertical surfaces.
Defects occur only in areas with thick application.

These phenomena lead people to suspect paint viscosity, thinner, or drying characteristics.
Paint or thinner can certainly be involved.

However, many cases of sagging and bubbles originate in application technique and film-thickness control, as well as the paint.

Two points are particularly important.

The first is applying too much paint at once.
The second is bringing the spray gun too close.

A thicker film is not necessarily better.
The important point is an appropriate thickness that allows solvent and air to escape readily and the surface to dry consistently.

1. What Is Sagging?

Sagging is a defect in which applied paint flows downward under gravity before it dries.

It commonly occurs on vertical surfaces, corners, and lower edges, taking the form of streaks, waves, or tears.

The basic cause is that the film cannot support its own weight before the paint dries.

Immediately after application, the film is still close to a liquid state.
If it is excessively thick in this condition, the paint flows downward readily.

Sagging is therefore a defect that occurs when the film is too thick and drying or viscosity increase cannot keep pace.

2. Conditions That Promote Sagging

The following conditions make sagging more likely.

• Excessive film thickness in one application
• Insufficient distance between the spray gun and workpiece
• Excessive output
• Application speed that is too slow
• Excessive overlapping application at the same location
• Paint viscosity that is too low
• Thinner that evaporates too slowly
• Insufficient setting time
• Low workpiece temperature
• Paint readily accumulating at vertical surfaces or corners
• An application method unsuited to workpiece geometry

Operators sometimes apply thick films to achieve hiding power or gloss, or to finish in one application.

However, even if a thick application looks good initially, it can cause sagging, bubbles, poor drying, after-tack, and pinholes.

Stable coating quality requires the approach of building an appropriate film thickness consistently over multiple stages, rather than achieving the entire required thickness at once.

3. Why Spraying Too Close Causes Sagging

Bringing the spray gun too close to the workpiece sharply increases the quantity of paint deposited per unit area.

This creates locally thick film.
These thick areas become heavy before solvent escapes and are more likely to flow downward.

At a short spray distance, the paint mist also has insufficient space to spread and concentrates locally.
This increases the likelihood of uneven coverage, rough finish, and localized sagging.

Spray coating requires a balance between gun distance, air pressure, output, and application speed.

When sagging occurs, first check the following before immediately suspecting the paint.

• Is the gun too close?
• Is output excessive?
• Is application speed too slow?
• Is there excessive overlapping application?
• Is paint accumulating at corners or lower edges?

Unstable application conditions cause sagging regardless of how good the paint is.

4. What Are Bubbles?

Bubbles are defects in which air or solvent vapor remains inside the coating film and appears as bubbles.

Bubbles may be visible immediately after application or develop during drying.
They may burst and appear as small holes or pinholes after drying.

The essential point is that air or solvent inside the film could not fully escape.

With a thin film and appropriate drying, internal solvent and air escape relatively easily.
However, excessive thickness, overly rapid drying, early surface hardening, or large amounts of bubbles in the paint make bubbles more likely to remain inside the film.

5. Conditions That Promote Bubbles

The following conditions make bubbles more likely.

• Excessive film build
• Excessively high viscosity
• Air entrained during paint stirring
• Excessively high application air pressure
• Spray distance that is too short
• The film surface drying too quickly
• Entering the drying oven too soon
• Insufficient setting time
• Excessive use of fast-evaporating thinner
• Moisture on the workpiece surface
• Moisture or oil in compressed air
• Insufficiently dried primer

The combination of excessive film build and rapid drying requires particular attention.

A thick film makes it harder for internal solvent to escape.
Rapid drying then hardens the surface first and traps solvent or air inside.
Gas attempting to escape afterward pushes up the surface, producing bubbles or pinholes.

Bubbles do not necessarily arise because the paint already contains bubbles.
They also frequently form during film development.

6. Film-Thickness Control Is Important for Both Sagging and Bubbles

Sagging and bubbles look different.

Sagging involves paint flowing.
Bubbles involve gas remaining inside the film.

However, they share an important cause.

That cause is excessive film build.

A thick application is more likely to sag.
It also makes solvent and air inside the film harder to release.
Drying is delayed.
The surface and interior dry differently.

This can lead to multiple defects, including sagging, bubbles, pinholes, solvent popping, after-tack, and poor drying.

Achieving the required film thickness is important.
However, applying more than the required thickness does not improve quality.
It increases the risk of defects.

Coating-film quality depends on consistently appropriate thickness rather than greater thickness.

7. Thinner Selection Also Affects Sagging and Bubbles

Thinner is important in sagging and bubble prevention.

Thinner does more than dilute paint.
It strongly affects drying rate, when the surface hardens, and how solvent escapes from inside the film.

If thinner evaporates too slowly, the film remains close to a liquid state longer and sags more readily.
If it evaporates too quickly, the surface may dry first, preventing internal solvent or air from escaping and causing bubbles or pinholes.

Suitable thinner conditions may vary between hot and cold weather and between the rainy and dry seasons.

In hot, humid environments such as Vietnam, review thinner selection according to the season and factory environment.

8. Do Not Underestimate Setting Time

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 surface settles, some solvent escapes, and leveling progresses.

With insufficient setting time, the film is heated while much solvent remains inside.
Solvent then evaporates rapidly in the drying oven, making bubbles or pinholes more likely.

For sagging prevention, setting time also allows the film to gradually stabilize and become less prone to flow.

However, excessively long setting time may cause dirt deposition or poor texture.
Standardize setting time according to the paint, film thickness, temperature, humidity, and drying conditions.

9. Points Requiring Particular Attention in Factories in Vietnam

Factories in Vietnam have several conditions that can promote sagging or bubbles.

First is high humidity.
High humidity makes moisture more likely on workpiece surfaces or in compressed air.
Moisture causes bubbles, whitening, or poor adhesion.

Next is production speed.
Prioritizing delivery dates or output can encourage thick application in one pass, shortened setting time, or early entry into the drying oven.
These practices increase the risk of sagging, bubbles, and pinholes.

Insufficient standardization of application work also matters.
Differences in gun distance, application speed, overlap, and output between operators cause film-thickness variations.
These variations cause sagging and bubbles to occur inconsistently.

Sagging and bubbles are not simply matters of operator skill.
They require standard conditions, training, equipment control, and environmental control.

10. Shop-Floor Checkpoints

When sagging or bubbles occur, check the following items.

Sagging Checks

• Are defects concentrated on vertical surfaces, corners, or lower edges?
• Does film thickness exceed the specified value?
• Is the gun too close?
• Is output excessive?
• Is application speed too slow?
• Is there excessive overlapping application?
• Is paint viscosity too low?
• Does the thinner evaporate too slowly?
• Is workpiece temperature too low?
• Is setting time appropriate?

Bubble Checks

• Do bubbles appear immediately after application or during drying?
• Is film thickness excessive?
• Is air being entrained during paint stirring?
• Is viscosity too high?
• Is air pressure too high?
• Is too much fast-evaporating thinner being used?
• Are products entering the drying oven too soon?
• Is setting time sufficient?
• Is compressed air free from moisture and oil?
• Has the primer dried sufficiently?

Sagging and bubbles need to be investigated separately in this way.
However, film thickness, gun conditions, thinner, and drying conditions are important to both.

11. Improvement Perspectives CoatureVietnam Can Offer

CoatureVietnam does not treat sagging and bubbles as paint problems alone.
We examine them through the 4M perspective.

Material: Materials

• Is the paint suited to the application?
• Is the specified thinner being used?
• Does the thinner’s drying rate suit the environment?
• Is the curing-agent mixing ratio appropriate?
• Is paint viscosity controlled?

Method: Methods

• Is gun distance standardized?
• Is application speed appropriate?
• Is film thickness per application appropriate?
• Is the specified setting time followed?
• Does the work assume excessive film build?

Machine: Equipment

• Is the spray gun in normal condition?
• Are the nozzle and air cap free from abnormalities?
• Is air pressure stable?
• Is compressed air free from moisture and oil?
• Does drying-oven temperature rise too rapidly?

Man: People

• Does gun distance vary between operators?
• Do operators mistakenly equate thick application with good coating?
• Can they identify signs of sagging and make corrections?
• Can they check drying conditions when bubbles occur?

Where necessary, Management also includes film-thickness standards, viscosity standards, setting time, drying conditions, and rules for recording defects.

Conclusion

Sagging and bubbles occur when excessive film build, spraying too close, gun conditions, thinner, setting time, and drying conditions combine, as well as through paint-related factors.

Sagging is paint flowing under gravity before the film dries.
Bubbles are air or solvent remaining inside the film because they could not fully escape.

Excessive film thickness is an important cause common to both.

A thicker film is not necessarily better.
The important point is stable drying and curing at an appropriate film thickness without difficulty.

Coating the Future with Science and Care
CoatureVietnam comprehensively checks paint, thinner, application method, and drying conditions, and helps establish coating conditions that the shop floor can reproduce.