Dirt contamination is one of the coating defects frequently encountered on the shop floor.

Small particles appear on the coated surface.
Foreign matter enters the film.
Dust-like matter adheres to the surface.
The finished surface becomes gritty.
Inspection finds small protrusions.

When these defects occur, many operations first check inside the booth.

Is the booth dirty?
Are filters clogged?
Is there dust on operators’ clothing or gloves?
Is air blow-off before coating insufficient?
Has foreign matter entered the coating material?
Are guns or hoses dirty?

These checks are, of course, extremely important.
However, the source of dirt contamination is not necessarily confined to the booth.

Dirt may enter from outside it.
Airflow throughout the factory building, entrances, floors, ceilings, upstream processes, transport routes, or workpiece storage areas may be responsible.

To substantially reduce these defects, examine the entire building as well as the booth.

1. Dirt contamination begins before coating

Dirt contamination does not occur only at the instant of application.

Foreign matter already on the workpiece surface appears as particle defects after coating.
A workpiece passing through a dusty area after surface preparation collects dust.
Uncoated products left beside a walkway for a long time collect airborne matter.
Dirt may also fall from carts or fixtures during transport.

Dirt contamination therefore originates in upstream processes and transport as well as inside the booth.

Check the following points.

• Where do workpieces travel after surface preparation?
• Is the uncoated-product storage area clean?
• Are uncoated products exposed to outside air or dust?
• Are transport carts or fixtures dirty?
• Is dust adhering to water or oil on workpiece surfaces?
• Is waiting time before coating excessive?

If workpieces are dirty before entering the booth, repeated booth cleaning will not reduce defects.

2. Examine outside air and dust entering through doors

Factory entrances can be major sources of dirt contamination.

Shutters open for product delivery.
Forklifts enter and leave.
Operators pass frequently.
Doors remain open to relieve heat.
Air enters from outside passageways.
Moist air enters during the rainy season.
Dust or sand enters during dry periods.

These conditions readily allow dust, insects, sand, and moisture into the factory with outside air.

Risk is particularly high if uncoated or freshly coated products are near entrances.

Check the following points around entrances.

• Are shutters or doors left open?
• Does outside air flow directly into the coating area?
• Are uncoated products stored near entrances?
• Does forklift traffic raise dust?
• Are wind protection or double doors needed?
• Is there scope for air curtains or simple partitions?
• Do surrounding roads, soil, or airborne sand affect the area?

Dirt contamination prevention requires examination of factory entrances as well as the booth entrance.

3. Transport raises dust from the floor

Factory floors also affect dirt contamination.

Accumulated dust, dried coating debris, metal particles, sand, paper scraps, or masking offcuts are raised by people, carts, and forklifts.

This dust may adhere to uncoated products.
It may enter the booth.
It may adhere to films before drying.

Dirty floors are more than a 5S issue.
They are a coating quality issue.

The following locations require particular attention.

• Uncoated-product storage areas
• Areas around booth entrances
• Transport routes after surface preparation
• Forklift aisles
• Areas near sanding or machining processes
• Masking work areas
• Fixture storage areas
• Waste storage areas
• Areas around oven exits

Even floor cleaning can raise dust if the method is unsuitable.
Using air blow-off to scatter floor dirt requires caution in coating operations.

Reducing defects requires a cleaning method that avoids raising dust, as well as a clean floor.

4. Dirt also accumulates on ceilings, beams, and lights

Dust overhead is often overlooked in coating operations.

Ceilings.
Beams.
Pipes.
Ducts.
The tops of light fixtures.
Conveyor rails.
Booth tops.
Curtain rails.
Cable trays.

Dust or coating mist can gradually accumulate in these locations.

Although usually inconspicuous, it may fall because of vibration, airflow, work, cleaning, or equipment inspection.
If it lands on uncoated products or workpieces being coated, it causes dirt contamination.

Suspect falling dirt particularly in factories where ceilings or beams have not been cleaned for a long time.

Check the following items.

• Has dust accumulated on top of light fixtures?
• Has dust accumulated on ducts?
• Is dirt falling from conveyor rails?
• Is anything falling from ceilings or beams?
• Are booth tops or curtain tops dirty?
• Does cleaning drop dust downward?

Look upward as well as at the floor when preventing dirt contamination.

5. Is dust entering from upstream processes?

Processes before coating, such as sanding, machining, deburring, welding, assembly, or unpacking, may generate dust or foreign matter.

Proximity to the coating area increases the risk of bringing this matter in.

Examples include the following conditions.

• Sanding and coating processes are close together
• Deburring dust becomes airborne
• Metal particles remain on workpiece surfaces
• Packaging generates paper or cardboard dust
• Oil or dirt from assembly adheres to the product
• Carts from upstream processes are dirty
• Dust is present on operators’ gloves or clothing

Cleaning the coating area alone will not reduce defects if upstream processes continue bringing dirt in.

A system is needed to prevent foreign matter from reaching the coating process.

Add a cleaning process between upstream work and coating.
Separate transport routes.
Partition dusty processes.
Use workpiece covers.
Clean carts.
Use separate operator gloves or clothing.
Standardize workpiece checks before coating.

Dirt contamination is also an issue involving the connection with upstream processes, rather than the coating process alone.

6. Poor supply air brings dirt into the booth

Supply air is important as well as exhaust in a booth.

Exhausting air requires an equivalent supply of replacement air.
Unsuitable supply air may draw contaminated air from outside or elsewhere in the factory into the booth.

Inadequate supply-air filters allow dust in.
Poor inlet placement creates turbulence.
An unsuitable supply and exhaust balance draws air in through unexpected locations.

Check the following items.

• Where does supply air enter?
• Are supply-air filters appropriate?
• Are filter replacement criteria defined?
• Is the area around the air inlet dirty?
• Is supply air insufficient for the exhaust?
• Does turbulence occur inside the booth?
• Is humid or dusty outside air entering directly?
• Is contaminated factory air being drawn in?

Reducing dirt contamination requires a consistent supply of clean air as well as exhaust.

7. Forklift and cart movements carry dust

Forklifts or carts moving inside the factory raise floor dust.
They may also carry dirt on their tires or wheels into the coating area.

Take particular care where forklifts frequently pass close to uncoated products.

Dust rises whenever a forklift passes.
Dirt adheres to cart wheels.
Vehicles from outside approach the coating area.
Carts pass near coated products.
Narrow aisles bring vehicles close to workpieces.

Such traffic routes cause dirt contamination or scratches.

Possible countermeasures include the following.

• Separate forklift routes from uncoated-product storage
• Limit access for outside vehicles
• Clean cart wheels
• Cover uncoated workpieces
• Clearly separate aisles from storage areas
• Improve floors that readily release dust
• Review cleaning frequency according to transport volume

Transport is more than logistics.
It also affects coating quality.

8. Workwear, gloves, and wiping cloths are also sources of foreign matter

Foreign matter carried by people cannot be ignored.

Dust on workwear.
Dust on gloves.
Fibers from wiping cloths.
Masking offcuts.
Paper dust.
Hair.
Dirt on shoe soles.

These may adhere to workpieces before coating or to coated surfaces.

Take particular care if operators enter the coating area wearing the same clothing used in upstream processes, warehouses, or outdoors.
Touching uncoated products with dirty gloves is also a risk.

Check the following items.

• Is dedicated workwear provided for the coating area?
• Are glove replacement rules defined?
• Are lint-shedding cloths being used?
• Is air blow-off used to scatter dirt from clothing?
• Do operators move between dusty processes and coating?
• Are masking offcuts scattered around?

Create a system that prevents foreign-matter entry rather than blaming people.

9. Incorrect air blow-off before coating can be counterproductive

Many operations use air blow-off before coating.

It effectively removes dust from workpiece surfaces.
However, incorrect use can disperse dirt instead.

Examples include the following.

• Blowing off in a dusty location
• Removed dirt adhering to nearby workpieces
• Raising dust from the floor
• Water or oil in the air
• Leaving workpieces idle after blow-off instead of coating promptly
• A long distance between blow-off and coating locations

Air blow-off is not simply a matter of blowing harder.

The location, direction, pressure, and timing are important.
Air quality is also important.

Water or oil in compressed air causes coating defects.
Air blow-off therefore requires air-quality control as well as being a dirt countermeasure.

10. A defect may originate somewhere other than where it is found

Dirt contamination is difficult because the location where a defect is discovered is not necessarily where the dirt adhered.

It was found during inspection.
However, dirt may have adhered in the uncoated-product storage area.
It may have adhered during transport after surface preparation.
Air blow-off may have raised it.
It may have entered through booth supply air.
It may have adhered while waiting before drying.

The discovery location alone therefore does not establish the cause.

Analysis must trace where the workpiece traveled.

After surface preparation.
Uncoated-product storage.
Transport routes.
Air blow-off location.
Coating booth.
Flash-off area.
Oven entrance.
Cooling area after drying.
Inspection area.

Check where foreign matter could adhere along this flow.

Examine dirt contamination as a process flow, rather than an isolated point.

11. CoatureVietnam’s checkpoints for dirt contamination

CoatureVietnam examines more than the booth when checking dirt contamination.

We use the following perspectives.

Building

Examine entrances, windows, outside-air ingress, ceilings, beams, lighting, floors, and walls.

Air

Check supply air, exhaust, booth airflow, factory pressure balance, and dust in outside air.

Upstream processes

Examine proximity to dust-generating processes such as sanding, machining, assembly, or unpacking.

Transport

Check carts, forklifts, routes, and uncoated-product storage areas.

Workpieces

Examine surface condition before coating, waiting time, covers, and air blow-off methods.

People

Examine workwear, gloves, wiping cloths, masking materials, and operators’ movement.

Equipment

Examine filters, ducts, booth cleanliness, fixture contamination, and conveyor contamination.

Dirt contamination does not necessarily have one cause.
Several small sources may act together.

That is why checking the entire building and workpiece flow is important.

Conclusion

Looking only at the booth is insufficient to reduce dirt contamination.

Booth cleaning, filters, operators’ gloves, and material filtration are important.
However, relying on these alone may overlook causes.

Does outside air enter through doors?
Does transport raise floor dust?
Has dust accumulated on ceilings or light fixtures?
Is dust brought in from upstream processes?
Is supply air contaminated?
Do forklifts or carts pass near uncoated products?
Do workwear or cloths shed foreign matter?
Does air blow-off disperse dirt?

Dirt contamination originates from the entire building, airflow, workpiece flow, and people’s movement, as well as inside the booth.

CoatureVietnam examines the entire process rather than isolated points.
Where does the workpiece travel, wait, contact air, and contact people or fixtures?
Tracing this flow helps find the actual source.

Coating the Future with Science and Care
Reducing dirt defects protects quality, reduces repeat work, and eases the burden on people working on the shop floor.