To get a supplier to quote exactly what you need instead of a generic offer, your PPGI coil inquiry must state the full specification chain: substrate grade and thickness, zinc or zinc-alloy coating mass, paint system chemistry, top and back coat thickness, RAL or custom color, gloss level, coil inner and outer diameter, coil weight range, dimensional tolerances, and the test certificates you require. If any one of those fields is missing, the supplier will fill the gap with its own default, and the quotation you receive will describe a coil that is convenient to produce rather than the coil your line actually needs. This article walks through every field, explains why it matters, and gives you a checklist you can paste into your next RFQ.
Why an Incomplete PPGI RFQ Produces a Generic Offer
Prepainted galvanized coil is a configured product, not a catalogue item. Two coils with the same nominal thickness and color can differ in substrate steel grade, yield strength, zinc coating mass, primer type, topcoat chemistry, back coat thickness, surface treatment and coil geometry. Each of those variables changes cost, formability, corrosion behavior and paint adhesion.
When a buyer sends only “PPGI coil, 0.5 mm, RAL 9002, 1200 mm wide,” the supplier must assume values for everything else. The result is usually a quotation built around the most economical combination the mill can schedule: a commercial-quality substrate, a light zinc coating, a standard polyester topcoat and a minimal back coat. That may be perfectly adequate — or it may fail at the roll former, at the bending brake, or in the field. The buyer cannot tell which, because the offer does not disclose the assumptions.
Verified general industry guidance: prepainted steel performance is a function of the whole coating system, not the paint alone. Substrate, metallic coating, pretreatment, primer, topcoat and back coat act together. Contract-specific requirements — the exact values you must receive — come from your own end-use, your customer’s specification, or the applicable standard edition named in your purchase order.
Field 1: Substrate Grade and Mechanical Properties
Steel grade designation
State the substrate grade explicitly. Common designations include commercial quality, structural grades, and drawing or forming grades. If your application involves bending, profiling or deep drawing, the grade and its mechanical properties matter more than the coating.
Mechanical property targets
Specify yield strength, tensile strength and elongation where the forming operation demands it. For simple trapezoidal roofing profiles, moderate elongation is usually sufficient. For tight radii, lock forming or drawn components, higher elongation and a suitable grade reduce the risk of coating cracking.
Thickness and width
Give base metal thickness, not just “total thickness.” Paint and zinc add to the measured total, and confusion between the two is one of the most frequent causes of dispute. Also state width and whether you need slit coil or full-width coil.
Field 2: Metallic Coating — Zinc Mass, Not Just “Galvanized”
“Galvanized” is not a specification. State the coating type and the coating mass per unit area, for example zinc coating on both sides expressed in grams per square meter, or the coating designation used in the standard you are buying to.
Decisions to make before you write the RFQ:
- Coating type: zinc, zinc-iron alloy, zinc-aluminium, or another alloy coating. Each behaves differently in cut-edge protection and in paint adhesion.
- Coating mass: heavier coatings generally improve corrosion resistance at cut edges and scratches, but can affect formability and weldability.
- Coating distribution: confirm whether the stated mass is per side, total both sides, or a minimum single-spot value.
- Surface condition: skin-passed, non-skin-passed, or with a specific surface roughness target for paint adhesion.
If your project also requires unpainted galvanized material for flashings, gutters or structural accessories, keep the two specifications separate and consistent. The GI coil range is a useful reference point when matching coating mass between painted and unpainted components on the same building.
Field 3: Paint System Chemistry
Paint chemistry drives durability, color stability and chemical resistance. Name the system you want rather than leaving it open.
| Paint system | Typical strength | Where buyers usually specify it | RFQ detail to include |
|---|---|---|---|
| Polyester (PE) | General exterior durability, good color range, cost-effective | Roofing and cladding in moderate environments | Resin type, topcoat DFT, gloss, RAL |
| Silicon-modified polyester (SMP) | Better chalk and fade resistance than standard PE | Exterior walls and roofs with longer appearance expectations | Resin type, topcoat DFT, gloss, warranty terms if any |
| PVDF / fluoropolymer | High color and gloss retention, strong chemical resistance | Architectural, coastal, industrial or high-visibility facades | Resin content percentage, topcoat DFT, primer type |
| Polyurethane (PU) | Good flexibility and abrasion resistance | Formed parts and some industrial applications | Topcoat DFT, gloss, flexibility test method |
| Epoxy primer / specialty primers | Adhesion and corrosion barrier | Demanding corrosive environments | Primer chemistry and primer DFT |
Where a standard is referenced, confirm the required edition and the exact values it demands. Standards are revised, and a quotation that meets an older edition may not satisfy the edition named in your contract.
Field 4: Top Coat and Back Coat Thickness
Dry film thickness (DFT) is measured in micrometres and should be stated separately for primer, topcoat and back coat. A single “coating thickness” figure is ambiguous and frequently the source of rejection at goods-in inspection.
- Primer DFT: affects adhesion and corrosion protection beneath the topcoat.
- Topcoat DFT: the primary barrier against UV, moisture and chemicals. State a nominal value and the acceptable minimum.
- Back coat DFT: often thinner than the topcoat. It protects the reverse face and can influence how the coil behaves during forming. If the reverse face will be visible, specify it as a finish, not just a protective coat.
- Measurement method: state how DFT will be verified and whether measurement is on the coil or on a sample.
For a joined view of how substrate and coating decisions combine across a painted coil program, the PPGI coil specification pages are a practical cross-check while you assemble your RFQ.
Field 5: Color, Gloss and Appearance
Color reference
Give a RAL number where the color is a standard shade, and state whether a visual match or a measured color difference is acceptable. Where the color is a brand-specific or custom shade, provide a physical sample or a measured target and define the acceptable color difference tolerance.
Gloss level
Gloss is usually stated in gloss units measured at a defined angle. Common bands include matt, low gloss, semi-gloss and high gloss. Gloss affects appearance, dirt retention and, in some environments, heat absorption. Specify the band and the measurement angle, and state the tolerance.
Surface appearance
Decide whether you need a smooth finish, a structured or wrinkle finish, or a printed or embossed effect. Also confirm whether the reverse side should be a matching color, a neutral backing coat, or a wash coat.
Field 6: Coil Geometry — ID, OD, Weight and Tolerances
Coil geometry is a production constraint, not a formality. If the coil does not fit your mandrel or exceed your crane and decoiler limits, the material is unusable regardless of coating quality.
| Field | What to state | Why it matters |
|---|---|---|
| Inner diameter (ID) | Exact mandrel size required | Must match your uncoiler or recoiler |
| Outer diameter (OD) | Maximum OD acceptable | Affects coil car, crane and line clearance |
| Coil weight | Target weight and acceptable min/max | Handling limits and line run length between joins |
| Thickness tolerance | Standard or tighter, with the basis defined | Affects roll forming setup and finished profile |
| Width tolerance | Standard or tighter, plus edge condition | Affects slitting yield and profile accuracy |
| Coil set / camber | Acceptable limits if your line is sensitive | Influences tracking and scrap rate |
| Eye orientation | Horizontal or vertical | Determines handling and loading method |
Also state the coil protection you expect: paper interleaving, edge protection, strapping pattern, moisture barrier and whether the coil will be shipped in containers or by other means. Packaging requirements belong in the RFQ, not in a later email.
Field 7: Test Certificates and Inspection Documents
State which documents must accompany the shipment and what each must contain. Typical requests include:
- Mill test certificate covering substrate chemistry and mechanical properties.
- Metallic coating mass results.
- Coating thickness results for primer, topcoat and back coat.
- Paint adhesion test results, such as bend, impact or cross-hatch, with the method named.
- Color and gloss verification results.
- Dimensional inspection results for thickness, width and coil weight.
Be specific about the test method, the sampling frequency and whether third-party inspection is required. “Certificate of conformity” alone is a weak requirement because it does not tell you what was measured.
PPGI Coil RFQ Checklist
Use this list as a template. Copy it into your inquiry and fill in every line.
- Base metal thickness, with tolerance basis defined.
- Width, edge condition, and whether full width or slit coil.
- Substrate grade and required mechanical properties.
- Metallic coating type and coating mass, with per-side or total basis stated.
- Surface treatment or passivation requirement.
- Pretreatment type if specified by your end customer.
- Primer chemistry and primer DFT.
- Topcoat chemistry and topcoat DFT.
- Back coat chemistry and back coat DFT.
- Color reference, RAL or custom, plus color difference tolerance.
- Gloss band and measurement angle.
- Surface finish and appearance requirements.
- Coil ID, maximum OD, target weight and acceptable weight range.
- Coil set, camber and other flatness limits if relevant.
- Packaging, interleaving and edge protection.
- Test certificates and inspection documents required.
- Applicable standard, edition, and the exact contract values.
- Quantity, delivery schedule and destination port or site.
- Any marking, labelling or traceability requirement.
Common Mistakes That Trigger Generic Offers
- Stating total thickness instead of base metal thickness. The supplier may interpret it either way, and the delivered coil may be thinner than expected once coating is accounted for.
- Writing “galvanized” without coating mass. This leaves the single most important corrosion variable undefined.
- Omitting the back coat. The reverse face is often assumed to be a minimal protective coat, which may not suit visible or exposed applications.
- Giving a color name without a reference. “Light grey” is not a specification.
- Ignoring coil ID and weight limits. A technically correct coil that does not fit your line is still a problem.
- Requesting certificates without naming the tests. A generic conformity document may not contain the data your customer needs.
- Referencing a standard without the edition. Different editions can carry different requirements.
- Leaving quantity and schedule vague. Order size and lead time expectations affect how a supplier prices and schedules the run.
How to Handle Price in the RFQ
Coating and substrate choices are the main cost drivers in a prepainted coil quotation. Heavier zinc coating, higher-performance paint chemistry, greater film thickness, tighter tolerances, special colors and additional testing all add cost. Coil size and quantity also influence price because of scheduling and yield.
Because raw material and coating costs move, ask for a dated quotation with a stated validity period and the pricing basis, rather than treating any figure as fixed. Request the quote to be broken down by the specification fields above so you can compare offers on equal terms. If two quotations differ significantly, the difference is usually in a field one supplier left unspecified.
A note on service life: no supplier can responsibly promise a fixed number of years for a coating or a building. Actual performance depends on the substrate, the metallic coating mass, the paint system, the local environment, how the material is formed and installed, and how it is maintained. Treat any specific year figure as conditional on all of those factors being defined and controlled.
Frequently Asked Questions
What is the single most important field in a PPGI coil RFQ?
There is no single most important field, but the most commonly omitted one is metallic coating mass. Without it, the corrosion performance of the finished product is undefined, and the supplier will choose a default that may not match your application.
Should I specify topcoat and back coat thickness separately?
Yes. They serve different functions and are often different values. Stating them separately allows accurate inspection at goods-in and prevents disputes about whether the delivered coil meets the agreed specification.
Do I need to name a standard in my RFQ?
If your project, customer or market requires compliance, name the standard and the edition, and list the exact values you require. Confirm the required edition with your customer or authority before issuing the purchase order, because editions change.
How do I compare quotations from different suppliers?
Ask each supplier to quote against the same completed checklist and to disclose the value assumed for every field. A quotation that answers every line can be compared directly; one that leaves fields open cannot.
Can I request a sample coil before placing a full order?
Many suppliers accept sample or trial coil requests, but the terms depend on the supplier and the specification. Include the request in your RFQ and confirm whether the sample is produced to the same specification as the production order.
What if my application needs a color that is not a standard RAL shade?
Provide a physical sample, a measured color target, or a brand reference, and define the acceptable color difference tolerance and the measurement method. Custom colors may affect minimum order quantity and lead time, so state your expected volume clearly.
Send a Complete RFQ, Get a Complete Quote
A well-built PPGI coil RFQ is simply a specification sheet with a question at the bottom. Fill in the substrate, coating, paint, geometry, tolerance and certificate fields, state your quantity and schedule, and ask for a dated quotation with a defined validity period. The supplier can then quote what you asked for instead of what is easiest to offer.
If you would like your specification reviewed field by field before you issue it, or if you want a quotation built against your own checklist, contact ChinaCNU Steel and share your requirements. Include as much of the checklist above as you can — the more complete the input, the more precise the response.
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