You convert coating weight (g/m², often written gsm) to dry film thickness (µm) by dividing the coating weight by the coating’s density, then multiplying by 1,000: µm = (g/m² ÷ density in g/cm³) × 1,000. For most polyester topcoats a density of roughly 1.2–1.5 g/cm³ is a reasonable planning figure, which puts a 20 µm DFT topcoat at about 24–30 g/m². Your mill certificate should normally show primer type and its own DFT or weight, topcoat DFT (and colour/RAL reference), backcoat DFT and type, plus the total nominal build-up. The exact figures are contract-specific: the certificate must match the purchase order and the standard edition named in it, not a generic table from the internet.
Why the Same Coating Is Quoted Two Different Ways
Paint is bought, applied and tested in two different worlds. The paint maker sells liquid or powder by mass. The coating line sets roller pressures and oven temperatures to hit a thickness, because thickness governs hiding, flexibility and corrosion barrier behaviour. Importers often receive quotations in gsm because that is how paint consumption is costed; engineers ask for microns because that is how film performance is specified and how most inspection gauges read.
Neither unit is wrong. The problem is when a specification mixes the two without stating density, or when a certificate reports “total coating 60 g/m²” with no split between primer, topcoat and backcoat. A single total number tells you almost nothing about how the film will bend, scratch or weather.
The Three Layers and What Each One Does
Primer
The primer is the adhesion and corrosion-inhibiting layer. On a hot-dip galvanized substrate (see our GI coil page for substrate context), typical primer chemistries include polyester-melamine, epoxy, polyurethane and chromate-free or chromate-containing corrosion-inhibiting types. Primer DFT is usually the thinnest of the three layers — commonly in the region of 5 µm, but this varies by system and must be confirmed against the supplier’s technical data sheet and your contract. Ask for the primer chemistry, not just “primer included”.
Topcoat
The topcoat carries colour, gloss and most of the weather-facing durability. Polyester (PE), silicon-modified polyester (SMP), polyvinylidene fluoride (PVDF) and polyurethane (PU) behave very differently outdoors, and the resin choice constrains how thick the film can be applied without cracking during forming. Topcoat DFT is normally the largest single contributor to total build-up.
Backcoat
The backcoat is applied to the reverse face. It is not decorative. Its jobs typically include protecting the reverse side from handling and storage corrosion, improving slip during forming, and — on some systems — providing a surface compatible with foaming adhesives or bonding. Backcoat DFT is often lower than topcoat and may be a single coat or a two-coat system. If the coil will be bonded, laminated or used in a sandwich panel, the backcoat specification becomes a functional requirement and should be stated explicitly.
Microns to GSM: A Worked Conversion Table
The table below is a planning aid only. Density varies by resin, pigment loading and colour — especially for dark and highly pigmented shades — so treat these as indicative and always reconcile against the paint supplier’s own density figure and the mill’s certificate.
| Layer (indicative) | Typical DFT range | Assumed density | Approx. coating weight | What to verify |
|---|---|---|---|---|
| Primer | ~5 µm | 1.2–1.5 g/cm³ | ~6–8 g/m² | Chemistry, chromate-free status, DFT on certificate |
| Topcoat (polyester) | 20 µm | 1.2–1.5 g/cm³ | ~24–30 g/m² | Resin type, RAL/colour, gloss target, DFT |
| Topcoat (higher-build) | 25 µm | 1.2–1.5 g/cm³ | ~30–38 g/m² | Formability limits, T-bend requirement |
| Backcoat | 5–10 µm | 1.2–1.5 g/cm³ | ~6–15 g/m² | Function (anti-corrosion / slip / bonding), DFT |
| Total build-up | ~30–40 µm | — | ~36–53 g/m² | Sum of layers, not a single headline number |
Note the arithmetic trap: buyers sometimes add “20 + 5 + 5 = 30” and then compare that 30 µm figure against a supplier’s “60 g/m²” and assume the supplier is generous. At 1.35 g/cm³, 30 µm is roughly 40 g/m². The two numbers are describing the same film. If they do not reconcile, ask which layer the gsm figure covers.
What Should Actually Appear on a PPGI Mill Certificate
A certificate that only states “coating: 60 g/m²” is not a specification document. A useful certificate for a PPGI coil order normally identifies:
- Substrate: steel grade, thickness, width, and the metallic coating beneath the paint — zinc coating mass per side, or zinc-aluminium, with the standard and edition referenced.
- Primer: chemistry type and DFT or coating weight.
- Topcoat: resin type, colour reference, gloss level, DFT or coating weight.
- Backcoat: type, colour if relevant, DFT or coating weight, and functional purpose where bonding is involved.
- Total paint build-up: stated as a sum, with units and whether it is per side or combined.
- Mechanical results: T-bend, impact, adhesion (cross-hatch or equivalent), pencil hardness, and gloss — with the test method named.
- Traceability: coil/heat number, line, date, and the standard edition applied.
Where a standard such as an ASTM, EN, ISO or GB/T document is cited, confirm the required edition and the exact acceptance values in your contract. Standards are revised, and a certificate referencing an older edition may not satisfy your end customer’s specification.
RFQ Checklist for Coating Specification
Send this list with your enquiry so the quotation and the eventual certificate can be compared line by line:
- Substrate: steel grade, thickness, width, coil weight, and zinc or zinc-aluminium coating mass per side.
- Primer: chemistry, target DFT, chromate-free requirement (yes/no), and whether it is stated on the certificate.
- Topcoat: resin family, RAL or colour code, gloss target and tolerance, target DFT, and colour-match method.
- Backcoat: chemistry, colour, target DFT, and the functional requirement (corrosion protection, slip, bonding, or a combination).
- Total paint build-up: state it in µm and g/m², and state whether values are per side.
- Forming and fabrication: expected bend radius, T-bend, profiling or roll-forming operations, and any post-forming cut edges exposed.
- Test methods and acceptance values: adhesion, impact, T-bend, gloss, hardness — with method references.
- Certificate content: request a sample certificate format before ordering so you know what data will be supplied.
- Standard and edition: name it, and confirm the edition in the contract.
- Packing and protection: interleaving, edge protection, moisture control for transport and storage.
Common Mistakes When Reading a Coating Specification
- Comparing gsm with µm without density. The two units are convertible, but only with the right density for that specific paint.
- Accepting a single total figure. “Total 60 g/m²” hides whether the backcoat is 5 µm or 15 µm — a large functional difference.
- Ignoring per-side versus combined. A “total” that combines both faces can look impressive while each face is thin.
- Assuming thicker is always better. Beyond a point, extra DFT can reduce flexibility and increase cracking risk during forming. The right thickness depends on the forming severity.
- Treating the backcoat as cosmetic. If the coil will be bonded or laminated, backcoat chemistry and thickness are performance-critical.
- Overlooking the metallic coating. Paint performance depends heavily on the zinc or zinc-aluminium layer beneath it; a premium paint system over an inadequate substrate will not compensate.
- Quoting a standard without the edition. “Complies with ASTM” is incomplete; the edition and acceptance values matter.
- Not checking colour and gloss on the certificate. These are measurable and should be reported, not assumed.
Cost Drivers, Not Price Quotes
Coating cost is driven by resin type, pigment (dark and highly saturated colours typically cost more), primer chemistry, total DFT, line speed and curing energy, and the metallic coating beneath. Because these inputs move, any price is only meaningful with a date and a defined specification attached. Request a dated written quotation tied to the exact layer-by-layer build-up you specify, so a future comparison is like-for-like.
FAQs
How do I convert gsm to microns for PPGI?
Divide the coating weight by the coating density, then multiply by 1,000. For example, 30 g/m² at 1.35 g/cm³ gives roughly 22 µm. Use the density from the paint supplier’s data sheet, not a generic value, for anything contractual.
What primer DFT should appear on the certificate?
There is no universal number. Primer DFT depends on the system and the corrosion requirement. What matters is that the certificate states the primer chemistry and its own DFT or weight, and that these match your purchase order.
Is a thicker topcoat always better?
No. Thicker films can improve barrier protection but may reduce flexibility and increase cracking risk during bending or profiling. The appropriate DFT depends on the forming severity and the exposure environment.
Can a certificate show only total coating weight?
It can, but it should not be accepted for a specification-sensitive order. Ask for the primer, topcoat and backcoat split separately, with units and per-side clarification.
Does the backcoat affect service life?
Yes, particularly where the reverse face is exposed, handled repeatedly, or bonded. Actual performance depends on substrate, metallic coating, paint system, environment, forming and maintenance — no single layer guarantees an outcome.
Which standard should I reference?
Reference the standard your end customer or project requires, and confirm the exact edition and acceptance values in the contract. Standards are revised over time, so the edition matters as much as the number.
Next Step
If you send us your target layer-by-layer build-up — primer chemistry and DFT, topcoat resin, colour and DFT, backcoat function and DFT, plus the substrate and metallic coating — we can respond against that specification rather than a generic headline figure. Include the standard and edition you need and the certificate fields your engineer expects to see. Send your coating specification to ChinaCNU Steel and we will confirm what can be quoted, what needs clarifying, and which values will appear on the mill certificate.
Related Products & Get a Quote
Related Products from ChinaCNU Steel
Need a price? Contact ChinaCNU Steel for a fast RFQ — Send your specifications, quantity and destination port for an order-specific quotation.