Every Monday morning the cold paint booth produced fisheyes on the first batches: 9 percent of parts rejected, and the fault vanished by Tuesday. The pattern pointed to the weekend cleaning, and the hunt found silicone polish used on the booth walls.
This case study covers the silicone source audit, the water-break test that found it, and the cleaning and surface prep routine that cut fisheye rejects to 0.3 percent. The same steps apply to any cold paint booth after cleaning or maintenance.
Common Mistakes and How to Avoid Them
| Mistake | Why It Happens | Practical Fix |
|---|---|---|
| 1. Silicone polish near the booth | Silicone becomes airborne and lands on parts | Use silicone-free cleaners only |
| 2. Cleaner residue on walls | Residue breaks the film on the next batch | Rinse walls and floor with water |
| 3. No tack cloth before spraying | Dust and a thin oil film stay on parts | Tack wipe every part before spraying |
| 4. Mold release on plastic parts | Release agents are silicone-based | Degrease and wash plastic parts first |
| 5. Compressor air contaminated | Oil mist from the compressor breaks the film | Check filters and air drier per shift |
| 6. Paint too thick | Poor flow forms crater-like defects | Hold 18-22 s on Ford cup 4 |
| 7. Booth airflow too weak | Mist settles onto the wet film | Keep booth exhaust balanced |
| 8. Cleaning rags reused | Silicone is spread, not removed | Use fresh rags for every booth cleaning |
| 9. No water-break test | Contamination is invisible until cure | Test a panel before every batch |
Best Practices
- Use only silicone-free products inside the booth
- Rinse cleaned walls and floors with water
- Tack wipe every part before spraying
- Run the water-break test before each batch
- Check compressor air filters and drier daily
- Log cleaning products, batches and fisheye counts
Implementation Roadmap
| Step | Frequency |
|---|---|
| Clean the booth silicone-free | Weekly |
| Run the water-break test | Per batch start |
| Check air filters and drier | Per shift |
| Tack wipe parts | Every part |
| Review the fisheye record | Per shift handover |
Fisheye prevention flow
- Clean - Silicone-free cleaner, water rinse. (Polished surfaces spread silicone.)
- Test - Water-break on a test panel. (A clean look can still carry silicone.)
- Tack - Wipe every part before spraying. (Skipped wipes reintroduce dust and oil.)
- Spray - 18-22 s Ford 4 at 20-25 cm. (Contaminated air ruins the batch.)
Fisheyes mean surface energy is broken by silicone or oil. | The water-break test finds contamination before paint is applied.
Cold paint booth working data
Reference values for solvent cold painting of hardware.
| Parameter | Reference | Why It Matters |
|---|---|---|
| Viscosity | 18-22 s, Ford cup 4 at 20°C | Thick paint flows poorly and craters |
| Thinner share | 10-20% by volume | Over-thinning runs; under-thinning craters |
| Booth temperature | 20-28°C | Cold paint is thick; hot paint dries fast |
| Booth humidity | 40-70% RH | High humidity blushes and weakens film |
| Spray distance | 20-25 cm | Close distance pools film |
| Air dry to tack | 30-45 min | Early handling damages the film |
| Water-break gate | Continuous film on panel | Break means silicone or oil |
Booth cleaning check card
| Step | Product | Check |
|---|---|---|
| Walls | Silicone-free cleaner | No polish used |
| Rinse | Clean water | No residue streaks |
| Floor | Water rinse | Dry before spraying |
| Air filters | Replace per schedule | No oil smell |
Fisheye cause map
| Symptom | Likely cause | First fix |
|---|---|---|
| Fisheyes after cleaning | Cleaner or polish residue | Water-break test, silicone-free cleaner |
| Fisheyes on plastic | Mold release agent | Degrease and wash parts |
| Fisheyes mid-shift | Compressor oil mist | Check filters and drier |
| Craters in thick paint | Viscosity too high | Thin to 18-22 s Ford 4 |
Case 1 - Monday fisheyes traced to booth polish in a hardware plant
Scenario. A hardware plant in southern China rejected 9 percent of Monday-morning batches with fisheyes, and the fault disappeared by Tuesday; cleaning crews had been polishing the booth walls and floor with silicone furniture polish.
Action. The polish was removed from the cleaning cabinet, walls were re-cleaned with a silicone-free degreaser and rinsed with water, and a water-break test panel became the batch-start gate.
Result. Fisheye rejects dropped to 0.3 percent within one week, and the cleaning log now lists every product used in the booth.
Case 2 - fisheyes on plastic zipper pulls from mold release in Vietnam
Scenario. A cold paint line in Vietnam sprayed plastic zipper pulls that showed fisheyes on every batch, even with a clean booth; the parts carried silicone mold release from the molding shop.
Action. A degreasing wash with a silicone-removing agent was added before painting, and parts were tack wiped at the booth entry.
Result. Fisheyes on plastic parts fell from 7 percent to under 0.5 percent, and the wash step became a written gate for molded parts.
Frequently Asked Questions
Why do fisheyes appear after cleaning?
Polish and cleaners leave silicone films that break the surface tension of the paint.
How do I find the silicone source?
Run the water-break test and audit every cleaning product that enters the booth.
What cleaner should the booth use?
Silicone-free degreasers, always rinsed with water afterward.
Why use a tack cloth?
It removes dust and the thin oil film before the paint lands.
What viscosity for cold paint?
18-22 seconds on a Ford cup 4 at 20°C.
Why do plastic parts fisheye?
Mold release agents are silicone-based and stay on the surface.
How do I check compressed air?
Blow a panel and look for craters; check filters and the air drier.
What if fisheyes persist?
Audit everything that touches the parts: rags, gloves, release agents and booth products.
What booth conditions?
20-28°C and 40-70% RH for solvent-borne cold paints.
What Would You Like to Solve?
Describe when fisheyes appear, which booth cleaning products are used and which parts fail; a technical review can isolate the silicone source and set the prep gates.