Powder Coating Masking and Film Thickness Inspection: RFQ Guide for Sheet Metal
Direct answer: A powder coating RFQ needs three numbers, not one. Dry film thickness (DFT) with tolerance — 60 to 80 microns interior, 80 to 120 exterior. Masking specification — which holes, threads and mating faces stay bare. Inspection method — gauge type, measurement points, and minimum acceptable reading. Missing any one of these is why coated parts get rejected at incoming inspection.
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The Three Numbers a Coating RFQ Is Missing
Most sheet metal RFQs specify finish as one line: “Powder coat, black, RAL 9005.” That is a colour, not a specification. The coater chooses thickness, masking and inspection, and parts arrive with a 45-micron reading on a face that should have been 60.
| Parameter | What to State | What Goes Wrong |
|---|---|---|
| DFT target | Min and max in microns + measurement standard | Coater optimises for speed; recessed faces fall under minimum |
| Masking spec | Which features stay bare, plug type, thread class | Powder enters threads; re-tapping leaves bare metal |
| Inspection method | Gauge, point count, location, minimum accepted | Report shows an average that hides a bare recess |
Dry Film Thickness: What Number to Specify
| Service Condition | Typical DFT | Basis |
|---|---|---|
| Interior, dry | 60-80 µm | Coverage and colour hiding; no corrosion load |
| Exterior C2-C3 | 80-120 µm | ISO 12944 corrosivity class |
| Coastal C4 | 120-150 µm with proper system | Thickness alone does not substitute for system choice |
| Post-forming parts | 60-80 µm, avoid upper range | Thick film cracks at tight bend radii |
Specify a minimum at defined points, not an average. A part can average 75 microns and still have a 40-micron recess. The number that matters is the minimum on the worst-case geometry. State the standard too: ISO 2360 eddy-current on aluminium, ISO 2178 magnetic on steel. If the RFQ is silent, the coater may measure on a different substrate and the readings will not compare.
Masking: Where the Specification Actually Lives
Masking gets the least attention and causes the most rejections. Four categories of feature need protection, each with a different method.
| Feature | Method | What to Specify |
|---|---|---|
| Threaded holes | Silicone threaded plug | Thread size, class (M6×1.0 6H), insertion torque |
| Plain bores | Silicone push plug or cap | Nominal bore + tolerance, depth of protection |
| Mating faces | High-temp polyester tape, 2 mil silicone adhesive | Area to remain bare; tape survives cure without residue |
| Grounding points | Tape or plug, plus verification | Electrical resistance after coating, acceptable ohm range |
The plug tolerance question. A silicone plug 0.2 mm undersize lets powder infiltrate the bore. After curing the thread needs re-tapping — which removes coating from the surrounding face and leaves bare aluminium as the first corrosion site. Oversize plugs deform the thread. Specify plug as nominal bore plus tolerance, and ask what insertion torque is controlled per batch.
Film Thickness at Masked Edges
At every masked boundary the film tapers from full thickness to zero. That width depends on tape adhesion, cure profile and powder charge.
If the masking line sits on a sealing face, the taper becomes a leak path. If it sits next to an assembly fit, it adds unintended interference. Define the line position with a tolerance — typically ±0.5 mm — not just name the area. Ask for taper zone width in the first article report.
Inspection: What the Report Must Contain
A report stating only an average is not a report. Specify these five items and the coater produces a usable document.
- Gauge type and calibration date — magnetic or eddy-current, with the standard followed
- Number and location of points — marked on a drawing or photo, minimum 5 per part
- Minimum and maximum reading — plus the specific point each came from
- Masked points list — which features were protected and by what method
- Taper zone width at each masking boundary
For production lots add a sampling rule: how many parts per lot, how many points per part, and the acceptance criterion. A common approach is 5 parts per lot at 5 points each, every reading above the specified minimum. That rule is testable. “Coating thickness as per standard” is not.
What VOLCRIX Handles, and What We Do Not
In-House and Coordinated Capability
- Stamping and forming — 40 presses 5-200 tons, progressive die capability
- Masking specification development — plug and tape selection per feature, documented
- Coating coordination — qualified partners, DFT verified to your standard
- Incoming verification — gauge readings at defined points, report per lot
- Dimensional inspection — Zeiss CMM on pre-coat and post-coat features
- First article — PPAP-style package with DFT map, masking record, taper measurement
We do not operate a powder coating line. Stamped parts are coated by qualified partners, and we hold the inspection responsibility and issue the DFT report. That distinction matters: if your program needs coating in-house under one roof with a single quality system, tell us at quoting stage.
About VOLCRIX
VOLCRIX has made precision metal components in Yueqing, Wenzhou since 1991 — 35 years. 40 stamping presses 5-200 tons, 60+ Japanese Star Swiss machines, 20 injection molding presses, 3/4/5-axis CNC.
ISO 9001:2015 and IATF 16949. Spectrometer per heat lot, Zeiss CMM. 500+ customers in 40+ countries.
Frequently Asked Questions
What dry film thickness should I specify for powder coating?
60 to 80 microns interior, 80 to 120 microns for exterior C3. Below 60 microns coverage gaps appear at edges and recesses; above 120 microns impact resistance drops and the film cracks on forming. State the range and the measurement standard, not a single number.
How is masking film thickness measured?
Two different things get confused. Coating DFT is measured with a magnetic gauge on steel or eddy-current gauge on aluminium at defined points. The masking material itself has its own thickness on the datasheet, typically 50 to 100 microns for polyester tape. Ask for DFT readings, not masking material thickness.
What happens if masking plugs are the wrong size?
Powder creeps into the gap. A plug 0.2 mm undersize lets powder infiltrate the bore, and after curing the thread needs re-tapping, which removes coating locally and leaves bare metal. Oversize plugs deform the thread. Specify the plug as nominal bore plus tolerance and ask what insertion torque is controlled.
Why does coating thickness vary across the same part?
Faraday cage effect. Electrostatic powder follows field lines and cannot penetrate deep recesses, so edges build thick while recessed faces stay thin. On a complex stamped bracket the spread across one part can exceed 40 microns. That is why DFT is specified as a minimum at defined points, not an average.
Should I ask for a coating thickness report with first article?
Yes, and specify the contents: gauge type and calibration date, number and location of measurement points, minimum and maximum reading, and which points were masked. A report stating only average thickness does not tell you whether a recessed face is bare.
Need Powder Coating + Masking on Stamped Parts?
DFT control · Silicone masking · Coverage report · Quote in 24h
Related Pages
- Custom Metal Stamping Parts
- Precision Stamping Tolerances
- Automotive Metal Stamping Parts
- Quality Control
Standards Referenced
- ISO 2360 — eddy-current coating thickness on aluminium
- ISO 2178 — magnetic coating thickness on steel
- ISO 12944 — corrosivity C1-C5, coating systems
- Qualicoat — powder coating approval for architectural aluminium
- ISO 9001:2015 / IATF 16949
