A rack paints differently at the top and the bottom, at the edges and in the centre. Coverage control is the intermediate skill that turns an acceptable rack into an even rack.
This guide covers gun path, rack position effects and the film map that shows where the rack is uneven.
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. Random gun path | Uneven film | Fixed pass sequence |
| 2. Top and bottom treated same | Film differs | Map the zones |
| 3. No film map | Evenness untested | Measure per zone |
| 4. Passes overlapped unevenly | Streaks | 50% overlap standard |
| 5. Rack angle ignored | Film runs to one side | Level and fix |
| 6. Booth airflow ignored | Film drifts with air | Set parts across the flow |
| 7. Changes in pairs | Cause unprovable | One variable per trial |
| 8. No film log | Tuning lost | Log the map |
Best Practices That Hold Up in Production
The operating disciplines that separate a reliable line from a reactive one.
- Use a fixed gun path and 50% pass overlap
- Map film by rack zone, not one spot
- Adjust one variable per trial
- Level racks and control airflow
- Keep the film map in the batch log
Implementation Roadmap
A practical sequence that can be adapted to your own project.
Process Flowchart
Film control flow
A step-by-step sequence with notes and cautions so every shift follows the same order.
- Top rows usually run heavier than bottom.
- Airflow moves film across the rack.
- Level the rack before mapping.
- Booth airflow must be set before tuning.
Working Data & Formula Notes
Film data
Reference values for rack film control.
| Component / Parameter | Working Value / Role | What Changes Mean (annotation) |
|---|---|---|
| Pass overlap | 50% | Less streaks, more even |
| Zones | Top, middle, bottom | Pattern shows in zones |
| Wet film | Per product window | Map, don't guess |
| Adjust | One variable | Proven changes |
Reference Data
Specifications and references cited in this guide. Confirm final parameters with your line supplier.
Zone film map
| Zone | Typical result | Action |
| Top rows | Heavier | Shorter passes or angle |
| Middle | Window | Standard |
| Bottom rows | Thinner | More passes or closer |
| Edges | Variable | Check overspray and air |
Implementation Cases
Case 1 - heavy tops and thin bottoms
Situation. A rack consistently painted heavy at the top and thin at the bottom; the map showed a clear gradient.
Approach. The gun path was adjusted with a shorter top pass and an extra bottom pass; the map was re-run.
Outcome. Top-to-bottom film levelled into the window; the map became the tuning tool.
Case 2 - streaks from uneven pass overlap
Situation. Vertical streaks appeared; passes were overlapped inconsistently by different operators.
Approach. A fixed pass sequence with 50% overlap was posted, and the film map verified the result.
Outcome. Streaks disappeared; the posted sequence standardized all operators.
Frequently Asked Questions
Why a fixed gun path?
Consistent passes make consistent film; random paths make streaks.
What overlap should I use?
About 50% pass overlap to avoid thin bands between passes.
Why map by zone?
The pattern - top, middle, bottom - shows where the rack is uneven.
How do I adjust the gradient?
Change pass length, angle or distance for the affected zone, then re-map.
Why level the rack?
A tilted rack lets film run to one side and falsifies the map.
How does airflow matter?
Booth airflow pushes overspray and shifts film across the rack.
Why one variable at a time?
Paired changes make the film response unprovable.
How often map?
Per batch type change and after any booth or gun change.
What goes in the log?
Pass sequence, zone film values and the change made.
What Would You Like to Solve?
Send us your rack layout, gun path and film measurements. We can help build the zone map and the pass sequence for even film across your rack.
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.