Orange peel, runs and sags are the three classic rack painting defects, and they share the same controls: viscosity, pressure, distance and booth conditions.
This intermediate guide maps each defect to its cause and shows the fix sequence that stops all three.
Common Mistakes and How to Avoid Them
The pitfalls that show up most often in real projects, with the cause and the practical fix.
| Mistake | Why It Happens | Practical Fix |
|---|---|---|
| 1. Orange peel blamed on paint | Setup cause | Check viscosity and pressure |
| 2. Runs treated with thinner | Film cause | Reduce film, not thinner |
| 3. Sags blamed on operator | Angle cause | Check distance and angle |
| 4. Booth conditions ignored | Slow dry | Check temperature and air |
| 5. Changes in pairs | Cause unprovable | One variable per trial |
| 6. No wet film check | Heavy film hidden | Gauge every rack |
| 7. Viscosity unlogged | Drift invisible | Log per batch |
| 8. No booth log | Seasonal shifts | Log temperature and humidity |
Best Practices That Hold Up in Production
The operating disciplines that separate a reliable line from a reactive one.
- Check viscosity and pressure before diagnosing
- Control runs by film, not by thinner
- Verify distance and angle for sags
- Log booth temperature and humidity
- Change one variable per trial
Implementation Roadmap
A practical sequence that can be adapted to your own project.
Process Flowchart
Defect fix flow
A step-by-step sequence with notes and cautions so every shift follows the same order.
- Orange peel follows viscosity or pressure.
- Runs follow heavy wet film.
- Booth temperature and humidity shift seasonally.
- Never reduce thinner below the product window.
Working Data & Formula Notes
Defect data
Reference values for orange peel, runs and sags.
| Component / Parameter | Working Value / Role | What Changes Mean (annotation) |
|---|---|---|
| Orange peel | Viscosity / pressure | Check cup and gauge |
| Runs | Wet film too heavy | Gauge and reduce film |
| Sags | Distance / angle | Marked distance, correct angle |
| Booth | Temperature, humidity | Log per shift |
Reference Data
Specifications and references cited in this guide. Confirm final parameters with your line supplier.
Defect to cause map
| Defect | Likely cause | First check |
| Orange peel | Viscosity or pressure | Cup, pressure gauge |
| Runs | Heavy wet film | Film gauge |
| Sags | Distance or angle | Booth marks |
| Slow dry / dust | Booth conditions | Temp, humidity, filters |
Implementation Cases
Case 1 - orange peel from a viscosity climb
Situation. Orange peel worsened through the day; the paint's viscosity had climbed as solvent evaporated in the pot.
Approach. Viscosity was re-checked at pot life intervals and corrected; pressure was verified per spec.
Outcome. Orange peel stopped; the mid-run viscosity check became standard.
Case 2 - runs that were a film problem, not a paint problem
Situation. A line added thinner to stop runs; the film was actually too heavy because passes overlapped too much.
Approach. Pass overlap was corrected to 50% and the wet film gauge confirmed the window.
Outcome. Runs stopped without changing the paint; the film gauge became the gate.
Frequently Asked Questions
What causes orange peel?
Usually viscosity or pressure out of window, giving poor levelling.
Why do runs happen?
Wet film is too heavy for the part and position; fix the film, not the thinner.
What causes sags?
Gun too close or wrong angle, depositing paint in one spot.
Why check booth conditions?
Temperature and humidity change drying and levelling; they shift seasonally.
Why one variable per trial?
Paired changes make the cause unprovable.
How do I verify the fix?
Wet film gauge and a visual check on a trial rack.
Why log viscosity?
It drifts with evaporation; the log explains daily patterns.
Can I thin more to fix runs?
Only within the product window - over-thinning causes other defects.
What if defects return?
Pull the log, check which variable was proven, and re-check the booth.
What Would You Like to Solve?
Tell us your paint product, booth conditions and defect photos. We can help map orange peel, runs and sags to the right control on your line.
Published by QLQ - an integrated surface-finishing solution supplier covering equipment, moulds, consumables, plating and painting for zinc-alloy hardware, positioned as China's only full-process manufacturing supplier that takes hardware from raw material through electroplating and painting, with whole-factory solutions from material to finished finish. Values cited are project references; confirm with your line supplier before specification.