Sagging, roughness and poor surface texture are frequent subjects of spray coating inquiries.

Coating material runs on the surface.
The surface feels gritty.
The film does not become smooth and develops an orange-peel-like texture.
Gloss is inconsistent.
The finish differs between areas.

When these defects occur, the shop floor may first suspect the coating material.
Its viscosity, resin design, pigment dispersion or thinner compatibility can indeed be the cause.

However, appearance defects in spray coating are not necessarily caused by coating material alone.

Even with the same material, a changed thinning ratio changes the finish.
A changed gun distance changes film thickness and texture.
A changed air pressure changes atomization.
An unsuitable thinner drying rate causes sagging or roughness.
Booth airflow that is too strong or too weak also affects the finish.

Thus, sagging, roughness and poor texture arise from the balance of application and drying conditions, as well as coating material.

1. Sagging occurs when the coating flows too much

Sagging is a defect in which coating material flows downward after application, leaving streaks or teardrop-shaped marks.
It occurs particularly on vertical surfaces, corners, edges and areas prone to excessive thickness.

The main cause is excessive flow of coating material on the workpiece.

Key points to check are the following.

• Is the material over-thinned?
• Is viscosity too low?
• Is too much applied in one coat?
• Is the gun too close?
• Is gun travel too slow?
• Are overlap areas becoming too thick?
• Is the thinner drying too slowly?
• Is flash-off time before drying too long?
• Is workpiece temperature too low?

Sagging tends to be attributed to a soft coating or slow thinner.
In practice, however, application conditions may be making the film too thick.

Even with the same material, applying several thin coats makes sagging less likely.
Conversely, applying too much at once can cause sagging with any coating material.

2. Roughness is likely when coating droplets dry in flight

Roughness is a defect in which the surface fails to become smooth and retains a fine granular texture.
It feels gritty and may also look less glossy.

A common cause is coating droplets drying too much before they reach the workpiece.

Check the following conditions.

• Is the gun too far away?
• Is air pressure too high?
• Is the thinner drying too fast?
• Is booth airflow too strong?
• Does the mist dry while traveling to the workpiece?
• Is the coating environment too hot?
• Is the coating being sprayed too thinly?
• Is overspray depositing again?

Roughness becomes likely when material deposits as dry particles before it can adequately wet and spread over the workpiece.

In hot environments such as Vietnam, thinner tends to dry faster and may cause roughness.
Excessively strong booth exhaust or supply air also carries the mist away and promotes drying before deposition.

When roughness occurs, check gun distance, air pressure, thinner drying rate and booth airflow alongside the coating material.

3. Poor surface texture occurs when atomization, viscosity and drying are out of balance

Poor surface texture is a defect in which the film does not become smooth and has an uneven appearance.
It can resemble orange skin.

It occurs when the material fails to level sufficiently on the workpiece.

The main causes are the following.

• Viscosity is too high
• Thinning is insufficient
• Atomization is coarse
• Air pressure is too low
• Gun distance is inappropriate
• The thinner dries too fast
• Leveling time after coating is insufficient
• Film thickness is insufficient
• The underlying surface texture is poor

Poor texture can appear to be the opposite of sagging.
Sagging is a defect caused by excessive flow.
Poor texture is a defect caused by insufficient flow and leveling.

Thus, in spray coating, both excessive and insufficient flow create defects.
This is the difficulty.

The coating needs enough thickness without sagging, while retaining enough flow to form a smooth surface.

4. Thinner does more than dilute coating material

Thinner is sometimes considered simply a means of diluting coating material in spray coating.
However, it is an important material that strongly determines the finish.

Thinner affects the following quality characteristics.

• Ease of atomization
• How the coating spreads
• Tendency to sag
• Drying rate
• Leveling
• Gloss
• Surface texture
• Roughness
• Risk of dirt deposition

A thinner that dries too fast makes roughness and poor texture more likely.
A thinner that dries too slowly makes sagging, pooling, drying defects and dirt deposition more likely.

The apparent drying behavior of the same thinner also changes with temperature and humidity.

Drying is faster in hot periods.
It may become inconsistent during high humidity.
Strong booth airflow makes coating material dry quickly.

Thinner selection therefore needs to consider shop-floor temperature, humidity, booth environment and required appearance, as well as coating compatibility.

5. Gun distance and air pressure affect both sagging and roughness

Gun distance and air pressure are strongly related to spray coating appearance defects.

For example, if the gun is too close, a large amount of material deposits in one place.
Sagging or excessive thickness then becomes more likely.

Conversely, if the gun is too far away, material dries readily before reaching the workpiece.
This causes roughness or insufficient thickness.

The same principle applies to air pressure.

Excessive air pressure makes mist too fine or causes excessive dispersion.
This may increase roughness, material loss and booth contamination.

Insufficient air pressure gives inadequate atomization and coarse droplets.
Poor texture and uneven thickness then become more likely.

Both gun distance and air pressure therefore need an appropriate range.
It is not simply a matter of higher pressure, shorter distance or longer distance being better.

6. Inconsistent film thickness makes appearance inconsistent

Sagging, roughness and poor texture are also closely related to film thickness.

Excessive thickness makes sagging, drying defects and solvent popping more likely.
Insufficient thickness makes roughness, poor hiding, insufficient gloss and poor texture more likely.

Thickness differences between areas also change appearance within the same product.

The following are examples:

• A good finish on flat areas but excessive thickness at edges
• Sagging only at corners
• Roughness only in recesses
• Different gloss only in overlap areas
• Areas where the underlying surface shows through

Such problems can occur.

Film thickness is fundamental to coating quality.
Appearance defect investigations need actual thickness measurements, rather than visual judgment alone.

7. The booth environment also affects the finish

The booth environment is important in spray coating.

The booth does more than exhaust coating mist.
It also needs to create conditions for consistent delivery of coating material to the workpiece and appropriate drying afterward.

Check the following items.

• Air velocity in the booth
• Balance between supply and exhaust air
• How airflow strikes the coating surface
• Temperature
• Humidity
• Dirt and dust
• Overspray accumulation in the air
• Operator standing position
• Workpiece arrangement

Excessive airflow can carry away coating mist and cause roughness or insufficient thickness.
Insufficient airflow allows mist to linger, causing dirt deposition or a poorer working environment.

High levels of booth dust or contamination also make dirt deposition on the film more likely.

When appearance defects occur, check the booth environment as well as the material and operators.

8. Consider the causes of sagging, roughness and poor texture separately

Sagging, roughness and poor texture are sometimes treated as similar appearance defects.
However, their causes differ.

They can be organized as follows.

Separating the defects this way makes causes easier to identify.

Treating everything as a coating material defect leads to inappropriate countermeasures.
Observe the defect carefully and separate the conditions involved.

9. A practical sequence for separating causes

When sagging, roughness or poor texture occurs in spray coating, the following sequence helps approach the cause.

1. Check the defect locations

The entire surface?
Only particular areas?
Vertical surfaces?
Corners?
Recesses?
Does it differ between operators?
Does it differ by time of day?

The location helps estimate the influence of operating conditions, shape and drying conditions.

2. Measure film thickness

Check thickness rather than judging only by appearance.
Are sagging areas too thick?
Are rough areas too thin?
Are there thickness differences in uneven-gloss areas?
Check these points.

3. Check viscosity and thinning ratio

Is viscosity within the standard range?
Does the thinning ratio differ between operators?
Is the correct thinner being used?
These checks are necessary.

4. Check gun conditions

Check air pressure, delivery rate, gun distance, gun travel speed and overlap width.
Differences between operators are particularly important.

5. Check drying conditions

Check thinner drying rate, booth airflow, temperature, humidity and oven conditions.
Look for excessive drying immediately after application or, conversely, drying that is too slow.

6. Check the booth environment

Check air velocity, dust, overspray and supply/exhaust balance.
The booth environment strongly affects roughness and dirt deposition.

10. Directions for improvement

The following approaches can improve sagging, roughness and poor texture in spray coating.

Standardize viscosity and thinning ratio

Quality cannot be consistent if operators thin by different intuitions.
Using viscosity cups or reference viscosities helps everyone work with similar material conditions.

Review the thinner

Drying that is too fast makes roughness and poor texture more likely.
Drying that is too slow makes sagging and drying defects more likely.
The thinner needs to suit the shop-floor environment.

Standardize gun distance

Inconsistent gun distance makes thickness and appearance inconsistent.
Besides training, practical ways to check distance on the shop floor are useful.

Review air pressure and delivery rate

Determine appropriate pressure and delivery while checking atomization and film thickness.
Avoid extremes that are too high, too low, excessive or insufficient.

Measure film thickness

Connect appearance defects to film thickness rather than judging by perception alone.
Thickness data make causes easier to separate.

Improve the booth environment

Check supply and exhaust air, velocity, dust and cleanliness.
An inconsistent environment prevents consistent quality even with appropriate operating conditions.

11. Support operator skill through condition control

Spray coating is strongly affected by operator skill.
However, excessive reliance on skill makes quality depend on individuals.

What matters is not blaming operators.
It is establishing conditions that let them consistently produce conforming products.

To achieve this, establish the following:

• Standard viscosity
• Standard thinning ratio
• Standard air pressure
• Standard gun distance
• Standard delivery rate
• Standard film thickness
• Standard drying conditions
• Procedures for checking when defects occur

These standards and procedures must be clearly defined.

This creates a foundation for operators’ experience to be useful.
Combining experience with standard conditions makes coating quality more consistent.

12. Conclusion: Interpret appearance defects through process conditions

Sagging, roughness and poor surface texture in spray coating are not just material problems.

Sagging results from excessive coating flow.
Roughness results from coating depositing while drying.
Poor texture results from the film failing to level.

Their causes differ.

When a defect occurs, it is therefore important to check the following conditions before changing the coating material.

• Viscosity
• Thinning ratio
• Thinner drying rate
• Air pressure
• Coating delivery rate
• Gun distance
• Gun travel speed
• Overlap width
• Film thickness
• Booth environment
• Drying conditions

Coating the Future with Science and Care

Science means interpreting defects through conditions rather than perception.
Care means creating processes that consistently produce conforming products, rather than relying only on operators’ intuition or effort.

CoatureVietnam supports spray coating improvements through application conditions, thinner, booths, drying and work standards, as well as coating materials.

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