When uneven or incorrect color occurs on the coating shop floor, paint is the first suspect.

Is the paint batch different?
Was the color matching poor?
Is the manufacturer’s quality inconsistent?

The cause can certainly lie with the paint.
In actual workplaces, however, more fundamental operating conditions may be responsible.

These are insufficient stirring and inadequate color-change cleaning.

The paint has not been mixed thoroughly.
Pigment remains at the bottom of the can.
The previous color remains in hoses or guns.
Containers and stirring rods are not cleaned sufficiently.
No first-piece inspection is performed after a color change.

Under these conditions, uneven color, incorrect color, gloss differences, inclusions, and cissing may occur even when the same paint is used.

Selecting paint alone is not enough to stabilize color.
How paint is mixed, used, cleaned out, and changed to the next color must be standardized.

1. Paint is not necessarily uniform inside the can at all times

Paint consists of resins, pigments, additives, solvents, and other components.

Pigments are important components that provide color.
They are dispersed in the paint, but may settle over time.

Paints containing high-density pigments are particularly prone to pigment settling at the bottom of the can.
Paint stored for a long time, paint stored in hot conditions, and low-viscosity paint may have changed in condition.

Using only the upper liquid layer in this state gives a pigment concentration lower than intended.
As a result, the color may look lighter, hiding power may be insufficient, or gloss may change.

Conversely, paint used toward the end of the can may have a higher pigment concentration and appear darker.

In other words, using paint while it is not uniform inside the can can cause color changes even within the same can.

2. Simply saying stir well does not provide shop-floor control

Operators are sometimes instructed to stir paint well before use.

However, this instruction is very vague.

For how many minutes should it be stirred?
Which tool should be used?
Is the bottom of the can being stirred?
Is the absence of remaining sediment checked?
How soon after stirring must the paint be used?
Is further stirring needed during use?

Unless these points are defined, the stirring condition varies between operators.

One operator stirs for only 1 minute.
Another stirs for 5 minutes.
Some check the bottom, while others mix only the surface.
Some use a mixer, while others mix manually with a rod.

Under such conditions, color consistency depends on the individual.

Stirring must be standardized in an SOP rather than left to experience or intuition.

3. Insufficient stirring causes gloss defects as well as uneven color

Uneven color is not the only problem caused by insufficient stirring.

Unless pigments and matting agents are uniformly dispersed, coating film appearance will be inconsistent.

Problems commonly caused by insufficient stirring are as follows.

• Uneven color
• Incorrect color
• Insufficient hiding
• Gloss variation
• Matting defects
• Dirt inclusions
• Contamination by sediment
• Viscosity changes during application
• Differences in the apparent film build
• Variation within a batch

In matte paints in particular, matting-agent dispersion affects gloss.
Applying paint while pigments or additives remain settled can cause localized gloss changes.

Stirring is therefore a fundamental task affecting overall appearance quality, not just color control.

4. Paint condition also changes during use

Paint does not only need to be mixed before use.

Its condition may also change with time during application.

Examples include the following changes.

• Pigments settle again
• Solvent evaporates and viscosity rises
• Temperature changes alter viscosity
• Weak paint circulation creates concentration differences
• Conditions differ between the tank and hoses
• Components clog the filter

Long operating periods and the use of large paint tanks require particular attention.

There were no problems at startup, but the color changes with time.
The finish differs between morning and afternoon.
The color differs between the beginning and end of the production run.
If these phenomena occur, paint control during use must also be checked.

It is important to consider stirring, circulation, viscosity checks, filter checks, and temperature control together.

5. Inadequate color-change cleaning directly causes color contamination

A color change requires complete removal of the paint previously used.

However, coating equipment and tools contain areas where paint tends to remain.

• Inside spray guns
• Around nozzles
• Cups
• Hoses
• Pumps
• Tanks
• Filters
• Stirring rods
• Paint containers
• Measuring cups
• Mixing containers

If the previous color remains in these areas, it contaminates the next color.

Switching from dark to light colors requires particular care.
Even a small residue of black, red, or blue can appear as a major color shift in white or yellow.

Inadequate color-change cleaning is not a paint quality problem.
However, the customer sees the result as an incorrect color.

Color-change cleaning must therefore be treated as an important quality-control operation.

6. Thinking it has been cleaned is not enough

For color-change cleaning as well, simply instructing operators to clean thoroughly is insufficient.

Which solvent should be used?
How many cleaning cycles are required?
Which components must be disassembled for cleaning?
How is residual color inside hoses checked?
Is the first paint discharged after cleaning discarded?
How is first-piece inspection performed?

Unless these points are defined, cleaning quality depends on the operator.

Particular attention is needed in the following cases.

• Frequent color changes
• High-mix, low-volume production
• Switching to light colors
• Products with demanding appearance requirements
• Color changes in powder coating
• Shared hoses or tanks
• Lack of visibility into a subcontractor’s cleaning conditions

Cleaning is more than a setup task.
It is an important process for protecting the quality of the next batch.

7. First-piece inspection after a color change is important

First-piece inspection should always be performed after a color change.

If cleaning is insufficient, the first few items may be contaminated by the previous color.
Missing these initial defects can affect the entire production batch.

First-piece inspection checks the following items.

• Does the color match the standard sample?
• Does the gloss look appropriate?
• Are there any inclusions or foreign matter?
• Is there contamination by the previous color?
• Is film thickness within the specified range?
• Is the coating surface texture normal?

Where necessary, the first coated items immediately after a color change may be designated for inspection or disposal.

This may appear to be a material loss.
However, considering the risk of mass-producing color-contaminated products, first-piece inspection is an important prevention cost.

8. Stirring and cleaning also relate to film thickness control

Insufficient stirring or cleaning affects film thickness control as well as color.

When insufficient stirring causes uneven paint concentration, coating film formation may be inconsistent even when viscosity appears the same.
Viscosity changes caused by solvent evaporation also change delivery rate and film thickness.

Clogging in hoses or nozzles due to inadequate cleaning disrupts the spray pattern.
This may lead to uneven film thickness, a rough finish, and gloss defects.

Even when the visible problem is uneven color, the actual cause may be uneven film thickness.
Film thickness must therefore be checked at the same time when investigating uneven color.

Stirring, cleaning, viscosity, and film thickness are not independent control items.
They all affect coating film consistency.

9. Color-change cleaning is also important in powder coating

Color-change cleaning is not an issue exclusive to solventborne coating.
It is also extremely important in powder coating.

In powder coating, powder remains in booths, hoses, guns, recovery equipment, filters, and other areas.
Residual powder of the previous color contaminates the next color.

Even small amounts of another color may stand out in white and light colors.
In metallic or special colors, contamination by particles may also affect appearance.

Items to check in powder coating are as follows.

• Booth cleaning
• Checks for residual powder inside hoses
• Cleaning inside guns
• Handling of recovered powder
• Filter cleaning
• Color-change sequence
• Cleaning level before light colors
• First-piece inspection

Powder coating offers environmental advantages, but inadequate color-change control causes color contamination and appearance defects.

10. Common stirring and cleaning issues in factories in Vietnam

Factories in Vietnam may commonly face the following issues with stirring and color-change cleaning.

• Stirring time varies between operators
• Stirring does not reach the bottom of the can
• No check for settling before use
• Reliance on manual mixing instead of a mixer
• No rules for further stirring during application
• The amount of added thinner is decided by intuition
• Color-change cleaning procedures are not documented
• Inadequate checks for residual color inside hoses
• No rules for first-piece inspection after a color change
• Subcontractor stirring and cleaning conditions have not been checked

Under such conditions, identifying the cause becomes difficult when uneven color or color contamination occurs.

The important point is not to blame operators.
It is to make operating conditions visible so that operators can reproduce the same quality.

11. Organize stirring and cleaning through the 4Ms

Insufficient stirring and cleaning problems become clearer when organized through the 4Ms.

Material: materials

Check paints, pigments, thinners, and cleaning solvents.

• Is the paint prone to pigment settling?
• Does the paint require stirring before use?
• Is the thinner compatible?
• Is the cleaning solvent appropriate?
• Is the color particularly susceptible to contamination by the previous color?

Method: methods

Check stirring methods, thinning methods, and color-change cleaning procedures.

• Has stirring time been defined?
• Is the stirring method standardized?
• Is sediment at the bottom of the can checked?
• Is the thinning ratio recorded?
• Have the number of cleaning cycles and cleaning procedure been defined?
• Are there rules for first-piece inspection?

Machine: equipment

Check mixers, spray guns, hoses, tanks, and booths.

• Is the mixer appropriate?
• Does color tend to remain inside the hoses?
• Is the inside of the gun easy to clean?
• Are tanks and containers easy to clean?
• Is the powder booth sufficiently easy to clean?

Man: people

Check variations in the work of operators, color-matching personnel, and inspectors.

• Do stirring methods differ between operators?
• Does the judgment that cleaning is complete depend on the individual?
• Has responsibility for checking the first piece after a color change been defined?
• Are the effects of insufficient stirring and cleaning taught?

12. Management perspective: SOPs and records prevent recurrence

Preventing insufficient stirring and cleaning requires SOPs.

SOP stands for Standard Operating Procedure.
It is a document for reproducing the same quality through the same procedure even when the operator changes.

A stirring SOP defines items such as the following.

• Checks before use
• Stirring time
• Stirring method
• Mixer conditions
• Checking the bottom of the can
• Thinning procedure
• Further stirring during use
• Viscosity checks
• Control of leftover paint after use

A color-change cleaning SOP defines items such as the following.

• Components to be cleaned
• Solvent to be used
• Number of cleaning cycles
• Whether disassembly for cleaning is required
• Checking residual color inside hoses
• First-piece inspection
• Criteria for judging cleaning complete
• Recording method

Documenting these items, providing training, and keeping records makes it easier to prevent recurrence of uneven color and color contamination.

13. Coating the Future with Science and Care

Uneven color caused by insufficient stirring or cleaning is common on the shop floor.
However, the cause must not be attributed solely to operator inattention.

Stir well.
Clean thoroughly.
Be careful.

Instructions like these alone will not stabilize quality.

A system that brings results closer to consistency regardless of who performs the work is needed.

Define stirring time.
Check the bottom of the can.
Define cleaning procedures.
Perform first-piece inspection.
Keep records.
Explain the reasons to operators.

This is what CoatureVietnam means by bringing science and care to coating.

Science means controlling uneven color through stirring, cleaning, film thickness, viscosity, and equipment condition rather than intuition.
Care means creating standards that allow operators to work without uncertainty, instead of requiring vague judgments from them.

The first step toward consistent color is mixing paint correctly and cleaning correctly.
The more fundamental the task, the greater the value of standardizing it.

Practical checklist

To prevent insufficient stirring and inadequate color-change cleaning, check the following.

• Is paint stirred sufficiently before use?
• Has stirring time been defined?
• Is the stirring method consistent between operators?
• Does settled pigment remain at the bottom of the can?
• Are there rules for further stirring during use?
• Are thinning ratio and viscosity recorded?
• Are color-change cleaning procedures documented?
• Are the hoses, guns, tanks, and containers requiring cleaning clearly identified?
• Is there a method for confirming that cleaning is complete?
• Is first-piece inspection performed after a color change?
• Is the cleaning level sufficient before switching to light colors?
• For powder coating, are the booth and recovery system sufficiently cleaned?
• Are operators taught the effects of insufficient stirring and cleaning?
• Can stirring and cleaning records be checked when uneven color occurs?

CoatureVietnam’s perspective

Preventing uneven color and color contamination requires standardized stirring, thinning, viscosity control, color-change cleaning, and first-piece inspection as well as paint quality checks.

CoatureVietnam supports stable appearance quality in factories in Vietnam through observation of shop-floor work, development of stirring and cleaning SOPs, review of color-change losses, and operator training.

Uneven color arises from the details of shop-floor work.
Organizing fundamental tasks scientifically is therefore a direct route to stable quality.