Most paint defects are set in the first two seconds of a spray stroke. The pressure is slightly high, the fan is slightly narrow, the distance is drifting, and the part that looked fine wet shows orange peel or sagging after cure. Gun setup turns painting from a talent into a repeatable process.
This guide covers the four controls every painter owns — air pressure, fan pattern, fluid flow and wet-film thickness — and how they map to the defects that cost rework. QLQ supplies automatic hand-spray, tumble spray and electrostatic painting machines for small hardware, and as China's only full-process manufacturing supplier from raw material through electroplating and painting, we tune the same parameters on the machines we install.
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. Pressure set by ear | Air pressure drifts with compressor load and hose size | Set from a gauge at the gun, not the wall regulator |
| 2. Fan too narrow for the part | Heavy centre coat, thin edges, tiger-striping | Match fan width to part width; overlap strokes by half |
| 3. Distance creeping | Closer means sagging and runs; farther means dry spray and orange peel | Hold 150-300 mm and check with a spacer or posture discipline |
| 4. Triggering inside the stroke | Heavy start and end of each pass | Start and stop the trigger off the part |
| 5. Viscosity changing during the shift | Paint thickens in the cup as solvent evaporates | Check Ford cup at the start and on refills; add thinners per the TDS |
| 6. No wet-film measurement | The operator cannot see thickness while spraying | Use a wet-film gauge after each pass until the habit is set |
| 7. Wrong needle or nozzle for the paint | PU, rubber-touch and lacquer need different atomisation | Match air cap, needle and nozzle to the paint TDS |
| 8. Skipping flash-off | Solvent trapped in the film pops or blisters in the oven | Hold flash time between coats at the booth temperature |
| 9. Gun not cleaned between colours | Dried paint in the air cap ruins atomisation | Clean the air cap and fluid tip at every colour change |
| 10. Ignoring booth airflow | Overspray hangs in still air and lands on wet parts | Check booth airflow and filters before blaming the gun |
Best Practices That Hold Up in Production
The operating disciplines that separate a reliable line from a reactive one.
- Set pressure with a gauge at the gun, in working conditions
- Check the fan pattern on a card before every shift
- Measure wet film after test strokes until the habit is set
- Hold the distance; use guides for repeat work
- Check viscosity at the start of the shift and on every refill
- Flash-off between coats per the paint TDS
- Clean the gun at every colour change and at the end of the shift
- Keep booth airflow and filters on a fixed schedule
Implementation Roadmap
A practical sequence that can be adapted to your own project.
Process Flowchart
Spray gun set-up flow
A step-by-step sequence with notes and cautions so every shift follows the same order.
- Change one variable at a time: pressure, then the card, then the part.
- Keep the same painter set-up standard across shifts so batches match.
- A dirty gun explains most mystery atomisation changes.
- Never spray toward people or with the booth interlock bypassed.
- Solvent vapours are a fire and health hazard; keep ventilation on.
- Wear respiratory protection matched to the paint and solvent.
Working Data & Formula Notes
Working data — spray parameters and their defect signatures
Reference values for spraying small zinc-alloy hardware. Confirm with the paint TDS and your gun manual.
| Component / Parameter | Working Value / Role | What Changes Mean (annotation) |
|---|---|---|
| Air pressure | 0.2-0.4 MPa (reference) | Too high gives dry spray and orange peel; too low gives coarse droplets and sagging. Set at the gun, not the wall. |
| Gun distance | 150-300 mm | Closer concentrates the coat and causes runs; farther loses atomisation and gives a dry, pebbled finish. |
| Viscosity (Ford cup) | Per paint TDS at booth temperature | Thicker paint needs more air and gives heavier film; thinner paint sags. Recheck on refills. |
| Wet film build | Measure with a gauge after passes | The control before cure. Too low fails coverage and gloss; too high sags, blisters or traps solvent. |
| Flash-off time | Per TDS between coats | Too short traps solvent; too long can soften the previous coat or collect dust. |
| Fan width vs part | Match fan to part, overlap 50% | A narrow fan on a wide part bands the coverage; a wide fan on a small part wastes paint and oversprays. |
| Cure window | 120-180 C metal, 20-60 min | The film reaches full properties only inside the window. Under-cure fails adhesion; over-cure yellows and embrittles. |
Reference Data
Specifications and references cited in this guide. Confirm final parameters with your line supplier.
Parameter to defect map
| Sagging or runs | Pressure low, distance close, viscosity low, heavy wet film | Raise pressure, hold distance, check viscosity, measure wet film |
| Orange peel | Pressure high, distance far, viscosity high, fan narrow | Reduce pressure, adjust fan, check viscosity |
| Dry spray or pebble | Distance far, overspray, booth airflow high | Move closer, adjust airflow |
| Solvent pop or blisters | Flash too short, film too thick | Extend flash, reduce build |
| Heavy bands at stroke ends | Triggering on the part | Trigger off the part, overlap passes |
Gun maintenance schedule
| Air cap cleaning | Every colour change | Soak in solvent, blow dry |
| Fluid tip and needle | Daily | Inspect for wear and dried paint |
| Air filter or regulator | Weekly | Drain water and check gauge accuracy |
| Pattern test | Every shift | Card test before production |
| Full gun service | Monthly | Seals, needle packing, body |
Implementation Cases
Case 1 — sagging that followed a compressor change
Situation. A hand-spray line painting large sliders started seeing runs on the bottom edges of parts, on and off for a week.
Approach. The wall gauge still read the old value, but pressure at the gun had dropped after a longer hose was fitted. The painter set pressure at the gun, re-checked the fan pattern, and re-measured wet film on test parts.
Outcome. Runs disappeared, and the plant added a gun-end gauge check to the daily start-up list.
Case 2 — orange peel that resisted extra thinner
Situation. A tumble-painted hardware line reported orange peel on the top faces even after adding thinner, which then caused sagging.
Approach. The real cause was a narrow fan and a drifting distance, not viscosity. The operator widened the fan, held 200 mm distance, and reduced fluid flow so wet film stayed in the target band.
Outcome. Orange peel cleared without the sagging that the thinner had introduced, and the process card was updated with the pattern setting.
Frequently Asked Questions
What air pressure should I use for small hardware painting?
Work from the paint TDS; 0.2-0.4 MPa is a common reference for spray painting hardware. Set it with a gauge at the gun in working conditions, because the wall gauge can read differently.
Why do I get orange peel?
Orange peel usually means poor atomisation: pressure too high, distance too far, viscosity too high, or a fan too narrow for the part. Change one variable at a time.
How do I stop sagging on the bottom edges?
Sagging is a wet-film problem: too much paint, too close, too low pressure, or paint too thin. Measure wet film, hold the distance, and keep pressure in the window.
How do I know the film will be thick enough?
Measure wet film with a gauge during spraying, then confirm cured film with a thickness meter. The wet-film target comes from the paint solids and the cured-film spec.
What is flash-off and why does it matter?
Flash-off is the wait between coats that lets solvent leave the film. Skipping it traps solvent, which pops or blisters in the oven — the classic solvent-pop defect.
How often should I check viscosity?
At the start of the shift and on every refill. Solvent evaporates from the cup as you work, so viscosity drifts within a shift even with a perfect mix.
Can one gun do PU, rubber-touch and lacquer?
A gun can, but each paint type has its own needle, nozzle and air cap recommendation. Match the set to the paint TDS; switching paints without changing the set is a common defect source.
Why does my pattern have tails or a heavy centre?
Tails usually mean a dirty or worn air cap; a heavy centre means pressure is low or the needle is set too far. Clean the cap and re-test on a card.
What is the right distance?
150-300 mm is the common working range for hardware, depending on paint and part size. Hold it constant — distance drift is the quiet cause of both sagging and dry spray.
How do I keep batches matching across shifts?
Write the settings on the process card: pressure at the gun, viscosity, fan, distance, wet-film band and flash time. Have each shift start with a pattern test and a test part.
What Would You Like to Solve?
If a paint defect keeps coming back on the same part, the fastest diagnosis is a pattern card, a viscosity cup and a wet-film reading — all before changing the paint. Share your gun set-up, paint TDS values and a photo of the defect, and we can map which control is drifting.
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.