To confirm a PPGI coil supplier can actually meet your specification, audit the coating line rather than the brochure: ask for the line configuration and curing method, the written coating build-up (primer and topcoat dry film thickness by side), substrate and metallic coating evidence, in-process thickness and cure controls, and traceable test reports tied to the offered coil. Then verify that the quoted values, tolerances and test methods match your RFQ, and make the critical ones contractually binding. The questions below are designed to separate a coil trader who can source to a drawing from a producer who can hold that drawing repeatedly.
Start with the specification, not the supplier
An audit is only meaningful against a defined requirement. Before visiting or requesting documents, write down what the coil must do: substrate grade and strength, zinc or zinc-aluminium metallic coating mass, paint system and chemistry, colour and gloss target, front and back coating thickness, surface finish, coil dimensions, and the tests that will be used for acceptance. This becomes the yardstick for every answer you receive.
Where you reference a standard — for example a product standard for continuously organic-coated steel flat products, or a test method for coating thickness or adhesion — confirm the required edition and the exact contract values with your technical or quality team. Standards set frameworks and default options; your purchase order sets the binding numbers. The same applies to any performance claim: actual durability depends on substrate, metallic coating, paint system, environment, forming and maintenance, so treat single-number lifetime statements with caution.
Questions that reveal coating line capability
Line configuration and process control
- What is the line type and coating application method? Ask whether paint is applied by roller coating and how many coating heads are available for primer and topcoat on each side. The answer determines whether a two-coat or three-coat system is genuinely producible.
- How is the strip cleaned and pre-treated before painting? Degreasing and chemical pre-treatment (or an equivalent conversion process) affect adhesion. Ask what is controlled and how it is monitored.
- How is curing controlled and recorded? Peak metal temperature is central to paint performance. Ask for the control method, the target window for the offered paint system, and how a record is kept per coil.
- How is line speed linked to cure and thickness? Speed changes affect film build and cure. Ask how the line compensates and what alarms or interlocks exist.
Substrate and metallic coating control
- Where does the substrate come from and how is it certified? Request the mill certificate for the base steel and confirm grade, thickness tolerance and mechanical properties.
- How is metallic coating mass verified? Ask whether it is checked by weigh-strip-coat or another method, and at what frequency. For galvanized substrate context, see GI coil.
- How is surface condition handled before painting? Ask about oil, oxidation, roll marks and how defective strip is diverted.
Thickness, colour and appearance
- How is dry film thickness measured and on what sample? Ask about instrument type, calibration, measurement locations and the acceptance rule for primer and topcoat.
- How is colour controlled? Ask for the colour standard, spectrophotometer or colourimeter use, tolerance and how metamerism is handled.
- How is gloss and surface finish controlled? Confirm the target range and measurement geometry.
Testing, traceability and release
- Which tests are routine and which are type or periodic? Typical routine checks include thickness, adhesion, bend or T-bend, impact, gloss and colour; salt spray and humidity tests are usually periodic or qualification tests. Ask for the method, frequency and acceptance criteria for each.
- Can test reports be tied to the coil or heat number? Traceability is the difference between a certificate and evidence.
- What happens when a coil fails? Ask for the hold, re-test and disposition process, and who authorises release.
Factory capability check: what to look at on site
A site visit should confirm that the documents describe the line you can see. Whether a visit is practical or not, the same evidence can be requested remotely.
- Coil handling and storage: covered storage, edge protection, and control of condensation risk before painting and before dispatch.
- Line instrumentation: thickness gauges, temperature measurement, coating weight control and their calibration status.
- Laboratory scope: which tests are done in-house, which are outsourced, and how third-party results are identified.
- Changeover discipline: how colour, paint batch and substrate changes are managed to avoid cross-contamination and off-spec transitions.
- Packing and protection: interleaving, edge protection, moisture barriers and markings, because transit damage frequently becomes a quality dispute.
Ask for a sample that is representative of the offered specification — same substrate, same paint system, same coating build-up — and for the test data that goes with it. A sample without traceable data is a demonstration, not evidence.
Decision table: what to ask, what to verify
| Topic | Question to ask | Evidence to verify | Make contract-specific? |
|---|---|---|---|
| Substrate | Which grade, thickness and tolerance? | Mill certificate; thickness measurements | Yes — grade and tolerance |
| Metallic coating | What coating mass and test method? | Coating mass records; method stated | Yes — minimum mass and method |
| Pre-treatment | What cleaning and conversion process? | Process description; control records | Usually process requirement |
| Paint system | Which primer and topcoat chemistry, how many coats? | Paint data sheets; system description | Yes — system identification |
| Film build | Dry film thickness by side, primer and topcoat? | Thickness records; measurement rule | Yes — minimums and tolerances |
| Cure | How is peak metal temperature controlled and recorded? | Set-point window; per-coil records | Yes — control requirement |
| Colour and gloss | What standard, instrument and tolerance? | Colour approval record; gloss data | Yes — target and tolerance |
| Adhesion and formability | Which bend/T-bend and impact tests, at what frequency? | Test reports with method and results | Yes — acceptance criteria |
| Corrosion testing | Salt spray or humidity test scope and duration? | Reports identified as qualification or periodic | If your application requires it |
| Traceability | Can reports be linked to coil or heat number? | Sample certificate and marking | Yes — documentation requirement |
For a broader view of the product and its options, see PPGI coil. Treat the table as a framework: the “make contract-specific” column reflects general good practice, not a rule that every buyer must apply identically.
RFQ checklist
Send this with your enquiry so that responses are comparable and audit-ready.
- Substrate grade, strength class, thickness and tolerance.
- Metallic coating type and minimum coating mass, with test method.
- Paint system: primer and topcoat chemistry, number of coats per side, nominal and minimum dry film thickness per side.
- Colour standard and reference, gloss range and measurement geometry, surface finish.
- Required mechanical and coating tests, methods, frequencies and acceptance values.
- Coil dimensions: inner diameter, outer diameter, maximum weight, width and edge condition.
- Packaging, marking, moisture protection and handling requirements.
- Documentation: mill certificate, test reports, traceability method, declaration of conformity if required.
- Inspection rights: pre-shipment inspection, third-party inspection, sample retention.
- Commercial terms: quantity, delivery basis, dated quotation validity and price basis (see below).
Common mistakes in a PPGI supplier audit
- Auditing the supplier, not the specification. Without a written requirement, every answer sounds acceptable.
- Accepting a single film thickness figure. Primer and topcoat, front and back, each need their own minimums and tolerances.
- Ignoring the substrate. Paint performance is built on the metallic coating and pre-treatment; a strong topcoat on a weak substrate is still a weak coil.
- Confusing qualification tests with routine tests. A salt spray report proves a sample was tested, not that every coil is tested.
- Leaving test methods unstated. Results are not comparable unless the method, specimen and acceptance rule are fixed.
- Overlooking packing and storage. Many disputes arise after the coil leaves the line.
- Treating price as the only variable. Cost drivers include substrate grade and metallic coating mass, paint chemistry and film build, line yield and scrap, coil size, testing scope and packaging. Ask for a dated quotation that states the specification and validity, and compare like with like rather than a single number.
Frequently asked questions
How many questions are enough for a supplier audit?
There is no fixed number. A focused set that covers substrate, paint system, film build, cure, testing and traceability is usually more useful than a long generic list, because each answer can be verified against evidence.
Can I audit a supplier without visiting the factory?
Often yes, in part. Request line description, control records, calibration status, test reports and a traceable sample. A site visit adds confirmation of handling, storage and laboratory practice, but document review can screen out mismatches early.
Should I specify dry film thickness or wet film thickness?
Specify dry film thickness for acceptance and let the supplier control wet film internally. Confirm the measurement method, instrument and where on the strip measurements are taken.
Are salt spray hours a reliable predictor of service life?
No. Salt spray is a comparative laboratory test under defined conditions. Real performance depends on substrate, metallic coating, paint system, environment, forming and maintenance, so avoid converting test hours into a promised service life.
How do I compare quotations from different suppliers?
Issue the same RFQ specification to each, require a dated quotation, and check that substrate, coating mass, paint system, film build, tests and packaging are identical. Any deviation should be stated explicitly, not hidden in a lower price.
Which standard should I cite?
Cite the standard your project or market requires, then confirm the edition and exact contract values with your technical team. The purchase order, not the standard alone, should carry the binding numbers.
Next step
If you have a specification or a draft RFQ, you can send it for a technical review and a dated quotation against your stated requirements. Share the substrate, metallic coating, paint system, film build, tests and coil dimensions, and note any points where you want clarification before committing. Start at ChinaCNU contact and include your acceptance criteria so the response can be checked line by line.
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