PPGI Coil Gloss Measurement: Method and Specification

Gloss on PPGI coil is measured with a gloss meter at a defined geometry — most commonly 60 degrees — after the instrument is calibrated against a certified reference tile, and the reading is expressed in gloss units (GU) against that standard. To answer your order question directly: write a nominal gloss target, a measurement geometry, a tolerance band, a test surface condition and a sampling plan into the purchase specification, and require the supplier to report measured values rather than describing the finish only as “matte” or “glossy.” A sample coil or colour chip is useful for visual reference, but it is not a specification, because gloss drifts with paint batch, curing, film thickness and substrate texture.

This article explains the method, gives typical industry ranges you can use as a starting point, and shows how to convert those ranges into contract language. Ranges quoted here are general industry guidance for orientation only; the binding values are always those written into your purchase order or agreed technical data sheet.

Why gloss is a specification item, not a sample impression

Gloss is a surface optical property. It depends on how much light the cured paint film reflects at a defined angle, which in turn depends on resin chemistry, pigment and filler loading, film thickness, curing degree, and the topography of the substrate beneath the paint.

That combination changes between production lots. A sample approved at the start of a project may have been coated on a different substrate roughness, at a different line speed, or with a paint batch that has since been reformulated. If the contract says only “as per approved sample,” a dispute becomes a comparison of two physical objects under undefined lighting — which is difficult to resolve fairly for either party.

Writing a number and a method into the specification does three things:

• It gives the producer a measurable target to control the line against.
• It gives the buyer an objective acceptance criterion at goods inspection.
• It gives both parties a defined path if a dispute arises.

Gloss also interacts with downstream processing. Roll forming, bending and profiling stretch the coating, and some appearance attributes shift after forming. If appearance matters on the finished panel rather than the flat coil, state where the measurement is taken. For background on how the coated product is built up from substrate and paint, see the PPGI coil product page.

The 60-degree gloss meter method

Instrument and geometry

A gloss meter illuminates the surface at a fixed angle and measures the reflected light within a defined aperture. The three common geometries are 20, 60 and 85 degrees. The 60-degree geometry is the general-purpose choice for most coated metal because it discriminates reasonably well across the mid-gloss range that dominates prepainted architectural and appliance products.

The 20-degree geometry is more sensitive at high gloss, and the 85-degree geometry is more sensitive at low gloss (sheen). Some specifications therefore require a secondary geometry when the 60-degree reading falls outside the instrument’s best discrimination zone. If your finish is deliberately very matte or very high gloss, ask whether a secondary angle should be added to the specification.

Step-by-step measurement on coated coil

1. Prepare a flat specimen. Cut a piece large enough to lie flat and cover the instrument aperture. Remove oil, dust, fingerprints and any protective film from the measured area using a method agreed with the supplier; aggressive solvents can alter the film.
2. Condition and calibrate. Allow the instrument and specimen to reach a stable temperature. Calibrate against the certified reference tile supplied with the meter, following the manufacturer’s interval. Record the calibration result.
3. Choose measurement locations. Measure at defined positions across the coil width — for example, near each edge and at mid-width — and at several positions along the length. Avoid areas with obvious handling marks, coating skips or roll marks unless the specification says otherwise.
4. Take repeated readings. Take a stated number of readings per location and record each one, not only the average. The spread is often more informative than the mean.
5. Report. State instrument model, geometry, calibration tile identity, number of readings, mean, minimum and maximum, specimen orientation, and the surface condition (coated side, flat, uncleaned or cleaned).

What affects the reading

• Surface topography of the substrate. A rougher metallic coating or a different zinc coating mass can raise or lower apparent gloss under a fixed paint film. Substrate selection is discussed on the GI coil page.
• Paint film thickness. Thicker films can read differently from thinner ones even with the same paint.
• Cure and line conditions. Under- or over-cure changes the surface.
• Directionality. Some finishes read differently along versus across the coil direction. Specify the orientation.
• Instrument and operator. Different meters and different operators produce different numbers. Method discipline matters as much as the target value.

Typical gloss ranges for matte, semi-gloss and high-gloss finishes

The table below is general industry guidance for orientation. Actual achievable values depend on the paint system, colour, substrate and producer’s process capability. Treat these as starting points for discussion, not as guaranteed values for any specific product.

Two cautions. First, gloss is not the same as colour; a colour match does not guarantee a gloss match, and vice versa. Second, gloss is not a durability indicator by itself. A high-gloss and a matte finish can both be appropriate for exterior use. Coating performance depends on substrate, metallic coating, paint system, environment, forming and maintenance, and no gloss value predicts service life.

How to write gloss into the purchase specification

Core clauses to include

• Nominal target and tolerance. For example, “60° gloss: 30 GU nominal, ±5 GU,” with the tolerance agreed as realistic for the paint system.
• Measurement geometry and instrument class. Specify 60° and, where relevant, a secondary angle.
• Measurement standard and edition. If you reference a gloss test standard, name it and confirm the required edition and exact test conditions with your supplier and any end customer. Standards are revised, and methods differ in specimen preparation and reporting.
• Specimen condition. Flat, coated side, cleaned per an agreed method, protective film removed.
• Sampling plan. How many coils, how many positions per coil, how many readings per position, and whether the mean or every reading must fall inside the band.
• Acceptance and reporting. What the mill test certificate or inspection report must show, and what happens if a result falls outside the band.
• Reference sample status. If a physical sample is retained, state clearly that it is for visual reference only and that the numeric clause governs.

Decision table: what to write for your application

RFQ checklist for gloss and appearance

Include the following in your enquiry so the quotation and technical data sheet can be compared like for like:

• Product and substrate: prepainted galvanized or other coated substrate, with steel grade and metallic coating requirements.
• Colour reference: system, code and edition, plus whether a physical approval sample is required.
• Gloss category and numeric target with tolerance, and the measurement geometry.
• Test standard and edition, and who confirms the exact test conditions.
• Paint system type and nominal film thickness, since these influence achievable gloss.
• Measurement location: flat coil, coated side, cleaned or uncleaned, orientation.
• Sampling plan and reporting format for the inspection certificate.
• Whether appearance after forming or profiling is also a requirement.
• Coil dimensions, coil weight, packing and marking, so the appearance clause sits inside a complete order.
• Any end-customer or project specification that overrides your own clauses.

If you are also sourcing the uncoated substrate separately, keep the two specifications aligned — the metallic coating on the base coil influences how the painted surface reads. The galvanized coil page covers substrate parameters in more detail.

Common mistakes buyers make with gloss

• Approving a sample instead of a number. “Match the approved sample” is not measurable and invites dispute.
• Omitting the geometry. A 60° reading and an 85° reading of the same surface are not interchangeable.
• Omitting the tolerance. A nominal value with no band is unenforceable in practice.
• Setting an unrealistically tight band. Over-specifying raises rejection risk and cost without improving fitness for purpose.
• Comparing readings from different instruments or operators. Cross-check the method before comparing numbers.
• Measuring over protective film, oil or handling marks. The specimen condition must be defined and followed.
• Confusing gloss with colour or with durability. These are separate attributes with separate acceptance criteria.
• Referencing a standard without the edition. Always confirm the required edition and exact contract values.
• Ignoring post-forming appearance. If the visible face is a formed panel, say so before the order is placed.

Frequently asked questions

Can I just ask for “matte” or “glossy” instead of a number?

You can describe the intent that way, but the contract should carry a numeric target, a geometry and a tolerance. Descriptive words mean different things to different producers and inspectors.

What gloss value should I write into my order?

Start from the appearance your end application needs, then ask the producer what band their process can hold reliably for that paint system and colour, and agree a target and tolerance both sides can measure. The value is application-specific; there is no universal correct number.

Is the 60-degree geometry always the right choice?

It is the usual general-purpose choice for prepainted metal. For very matte or very high-gloss finishes, a secondary geometry may discriminate better. Decide this with your supplier and, where relevant, your end customer.

Does gloss affect how long the coating lasts?

Gloss is an appearance attribute, not a life prediction. Actual performance depends on substrate, metallic coating, paint system, environment, forming and maintenance. Do not use a gloss value as a proxy for durability.

What if the delivered gloss is outside the agreed band?

That depends entirely on the acceptance and remedy clauses in your contract. Define in advance whether the criterion applies to the mean, the minimum or every reading, and agree the inspection and remedy route before shipment.

Do I need to reference a standard at all?

Referencing a recognised test method makes the measurement reproducible, provided the edition and conditions are confirmed. If you reference one, confirm the required edition and exact contract values with your supplier.

Cost drivers and quotation notes

Gloss requirements can influence cost. Tighter tolerance bands, non-standard finishes, additional inspection and reporting, and secondary geometry testing all add work. Paint system selection, film thickness, substrate grade and coil dimensions are separate cost drivers. No numeric price is stated here because prices move with raw materials, energy and freight. Request a dated quotation that states the gloss target and tolerance you intend to contract, so the offer and the specification match.

Next step

If you are preparing an enquiry, send your colour reference, gloss target with tolerance, measurement geometry, sampling expectations and any end-customer specification. The team can respond with a technical data sheet and a dated quotation against those clauses. Use the ChinaCNU contact page to start the conversation, and state clearly which values are mandatory and which are open for discussion.

Related Products & Get a Quote

• PPGI Coil (Prepainted Galvanized)
• GI Coil (Hot-Dip Galvanized)
• Galvanized Steel Coil

Need a price? Contact ChinaCNU Steel for a fast RFQ — Send your specifications, quantity and destination port for an order-specific quotation.