Sep 28, 2026 · 12 min read

PPGL Galvalume Prepainted Coil Specification Guide

The specification points that matter most when ordering PPGL (prepainted galvalume) coil are: the metallic coating mass and designation (AZ), the steel substrate grade and mechanical properties, the paint system chemistry and dry film thickness, the surface treatment and pretreatment, the coil dimensions and tolerances, and the inspection documents that prove all of the above. Get any one of these wrong and the coil may still look correct on delivery while failing earlier than expected in service. This guide explains what each point means, where buyers commonly get it wrong, and how to write a specification that a mill can actually confirm in a contract.

One framing note before the details: the guidance below reflects general industry practice and common standards. Your project may require different values. Always confirm the required standard edition and the exact figures written into your purchase contract, because those are the numbers the supplier is obligated to meet.

What PPGL actually is, and why the substrate changes the paint question

PPGL is a coil product made by coating steel with an aluminium-zinc alloy (commonly 55% aluminium, 43.4% zinc, 1.6% silicon by mass) and then applying an organic paint system over that metallic coating. The metallic coating is usually described as AZ, followed by a number indicating coating mass in grams per square metre total both sides, for example AZ150.

Compared with PPGI (prepainted galvanized, zinc coating, usually designated Z), the Al-Zn coating offers different corrosion behaviour and better heat reflectivity, but it also behaves differently under paint. That difference is the single most misunderstood part of PPGL specification, and it is worth understanding before you write the paint clause.

Why painting over Al-Zn is not the same as painting over zinc

Aluminium-zinc coatings form a surface oxide that is different in character from the oxide on a pure zinc coating. Paint adhesion depends heavily on how that surface is prepared. In practice this means:

Pretreatment chemistry matters more. The conversion coating or pretreatment selected for an Al-Zn surface should be specified as suitable for Al-Zn, not simply inherited from a galvanized line. Ask the supplier to state the pretreatment type and that it is intended for the Al-Zn substrate.
Adhesion testing is not optional. Bending, impact and tape-adhesion results on the finished coil are the practical evidence that the paint system is bonded. Specify the test method and acceptance value you require.
Cut edges behave differently. The Al-Zn coating provides sacrificial protection, but the way it protects a cut edge differs from zinc. This affects how you design and specify for cut-edge exposure.
Forming can stress the coating. Severe bending or profiling can crack paint if the system is not flexible enough for the intended bend radius. Specify the minimum bend radius your process needs.

If you are comparing the two families in more depth, our pages on PPGI coil and galvalume steel coil explain the base products and how each is normally supplied.

The specification table: what to write and why

The table below lists the main specification points, what each one controls, and typical ways they are expressed. Treat the “how it is expressed” column as a starting point for discussion, not a fixed value.

Specification point	What it controls	How it is usually expressed
Metallic coating	Corrosion protection from the Al-Zn layer	AZ designation plus coating mass, e.g. AZ150, stated as g/m² total both sides
Substrate grade	Strength, formability, weldability	Commercial quality or structural grade with yield/tensile requirements
Substrate thickness	Load capacity, stiffness, weight	Base metal thickness, with tolerance range and whether coating is included
Paint system	Colour, weather resistance, chemical resistance	Primer type, topcoat chemistry (e.g. polyester, silicon-modified polyester, PVDF), and dry film thickness per side
Surface treatment	Paint adhesion and handling protection	Pretreatment type plus optional protective film or oil
Colour	Appearance and solar performance	Reference to a colour standard or a physical sample, with tolerance
Coil dimensions	Line compatibility	Width, inner diameter, outer diameter, coil weight range
Mechanical properties	Forming and structural behaviour	Yield strength, tensile strength, elongation, bend test
Inspection	Proof of conformity	Mill test certificate, coating mass test, adhesion test, dimensional report

Coating mass: the number buyers most often leave vague

The AZ number is not a paint thickness and it is not a total product weight. It describes the mass of the aluminium-zinc coating. Two conventions cause confusion:

Total both sides vs per side. A designation may be stated as total coating mass for both surfaces combined, or as a per-side figure. Always confirm which convention applies in your contract.
Minimum vs nominal. A nominal figure and a guaranteed minimum are not the same. For corrosion-critical work, specify the minimum you are willing to accept and require that it be verified by test.

Higher coating mass generally means more sacrificial protection, but it also affects forming behaviour and cost. There is no universal “best” number; the right value depends on the exposure environment, the expected service conditions, and how the coil will be formed and installed. Ask for the coating mass to be confirmed on the mill test certificate rather than assumed from the product name.

Confirm the standard edition

Coating mass, tolerance and test methods are often defined by reference to a standard. Standards are revised, and different editions can carry different values. When you name a standard in an RFQ, also state the edition and the exact acceptance values you require, and ask the supplier to confirm which edition they will work to. Do not rely on a standard number alone to communicate your requirement.

Substrate grade and mechanical properties

Paint and coating get most of the attention, but the steel underneath determines whether the coil survives your roll forming line and whether the finished panel performs structurally.

Questions to settle before ordering

Is the application structural or mainly architectural? Structural use usually requires a specified minimum yield strength and a grade that can be traced.
What is the tightest bend radius in your forming process? This drives the required elongation and the paint flexibility requirement.
Will the material be welded, riveted or fastened only? Grade and coating both affect joining methods.
Is base metal thickness or total thickness being quoted? These differ once coating and paint are applied, and the difference matters for load calculations and for roll former settings.

Write the grade, the mechanical property requirements and the thickness convention into the RFQ. If the supplier quotes a thickness without stating the convention, ask for clarification before comparing offers.

Paint system: chemistry, thickness and colour

The paint system is normally described in three parts: primer, topcoat and, where applicable, a back coat.

Chemistry

Different resin chemistries suit different exposures and budgets. Common families include polyester, silicon-modified polyester and fluoropolymer types such as PVDF. Each has different strengths in colour retention, chalk resistance and chemical resistance. The correct choice depends on the environment (coastal, industrial, inland), the expected maintenance regime, and the appearance requirements of the project. Rather than accepting a generic “polyester” label, ask the supplier to state the resin chemistry and the intended application class.

Dry film thickness

Paint performance depends on dry film thickness, usually expressed in micrometres per side, with primer and topcoat stated separately. Specify a minimum dry film thickness and require it to be verified. Note that thicker is not automatically better for every application: excessive thickness can reduce flexibility and affect forming. The right value balances durability with the forming the coil will undergo.

Colour and appearance

Colour should be defined against a recognised colour reference or a retained physical sample, with a stated tolerance and a stated viewing condition. Also specify whether the back side needs a full coat, a thin protective coat, or a specific colour, because back-coat specification affects both cost and appearance.

Surface treatment, handling and storage

Surface treatment sits between the metallic coating and the paint, and it is where many adhesion problems originate if it is specified loosely.

Pretreatment: confirm the type and that it is appropriate for an Al-Zn surface.
Protective film: if a strippable film is applied, specify its type and a maximum storage period before removal, and confirm removal instructions.
Oiling: some orders specify a protective oil; confirm compatibility with your forming and any downstream painting or bonding.
Storage and handling: moisture trapped between coils, prolonged storage in humid conditions, and poor lifting practice can all damage prepainted surfaces. Ask for the supplier’s handling and storage recommendations and pass them to your warehouse.

RFQ checklist for PPGL coil

Use this list to make your enquiry complete. A complete RFQ reduces the chance of comparing offers that are not actually comparable.

Product: prepainted galvalume (PPGL) coil, stated clearly.
Metallic coating: AZ designation, coating mass, and whether the figure is total both sides or per side.
Standard reference: standard number, edition, and the exact acceptance values required.
Substrate: grade, minimum yield strength if structural, elongation requirement.
Thickness: base metal thickness, tolerance, and the thickness convention.
Width, inner diameter, outer diameter and coil weight range.
Paint system: primer type, topcoat chemistry, back-coat requirement, dry film thickness per side.
Colour: reference standard or sample, tolerance, gloss level if relevant.
Surface treatment: pretreatment type, protective film or oil, storage limits.
Forming requirements: minimum bend radius, profiling method, any drawing or deep-forming operation.
Inspection documents: mill test certificate, coating mass test, adhesion test, dimensional report.
Packaging: eye-to-sky or eye-to-wall, edge protection, moisture protection, marking and labelling.
Quantity, delivery schedule and destination port or site.
Any application-specific requirements: coastal exposure, chemical exposure, fire or thermal considerations.

Common specification mistakes

Specifying “AZ” without a number or a convention. The supplier cannot confirm what was not stated, and you cannot verify what was not agreed.
Copying a PPGI specification onto a PPGL order. Pretreatment and paint adhesion requirements are not automatically transferable between zinc and Al-Zn substrates.
Quoting thickness without stating the convention. Base metal thickness and total thickness produce different performance and different prices.
Naming a standard without an edition or acceptance value. Editions change, and a standard number alone does not define your requirement.
Ignoring the forming process. A paint system that passes a flat-panel test can still crack at a tight bend radius.
Leaving colour tolerance undefined. “Blue” is not a specification; a colour reference with a tolerance is.
Not specifying inspection documents. Without agreed test certificates, conformity is difficult to demonstrate after delivery.
Treating price as the only comparison point. Offers with different coating mass, paint thickness or grade are not equivalent, even when the headline figure looks similar.

Cost drivers and how to request a quote

PPGL coil cost is driven mainly by the metallic coating mass, the substrate grade and thickness, the paint chemistry and dry film thickness, the colour and any special finish, the coil dimensions and weight range, the quantity, the packaging, and the delivery terms. Because these inputs vary between orders and because raw material and freight conditions change, a meaningful price must be tied to a specific specification and a specific date.

When you request pricing, send the full specification from the checklist above and ask for a dated quotation that states the validity period, the specification it is based on, the delivery terms, and the inspection documents included. If a supplier cannot state the coating mass convention or the paint chemistry in writing, that is a gap worth resolving before you compare numbers.

Frequently asked questions

What does AZ150 mean on a PPGL coil?

AZ indicates an aluminium-zinc metallic coating, and the number indicates coating mass. However, conventions differ on whether the figure is total for both sides or per side, and whether it is nominal or minimum. Confirm the convention and the acceptance value in your contract rather than assuming.

Can I use the same paint specification for PPGI and PPGL?

Not automatically. The Al-Zn surface behaves differently from a zinc surface during pretreatment and painting, so the pretreatment chemistry and adhesion requirements should be specified for the Al-Zn substrate. Reusing a PPGI paint clause without review is a common source of adhesion problems.

How long will a PPGL roof last?

There is no single answer. Service performance depends on the substrate grade, the metallic coating mass, the paint system, the local environment, how the material is formed and installed, and how it is maintained. Suppliers should not promise a fixed service life, and buyers should evaluate specifications against their specific exposure conditions rather than relying on a generic number of years.

What paint thickness should I specify?

Specify a minimum dry film thickness per side, with primer and topcoat stated separately, and align it with the exposure environment and the forming the coil will undergo. Excessive thickness is not automatically better, because it can affect flexibility. Confirm the value with your supplier against your application.

Which documents should come with the coil?

Typically a mill test certificate covering the substrate and coating, results for coating mass and paint adhesion, and a dimensional report. The exact document set should be agreed in the contract, since requirements differ between projects and destinations.

Does a higher coating mass always mean better performance?

Higher coating mass generally provides more sacrificial protection, but performance also depends on the paint system, the environment, edge detailing, forming quality and maintenance. Coating mass is one variable among several, not a standalone guarantee.

If you are preparing an enquiry and want your specification reviewed against these points, you can send your requirements through our contact page. Include the coating mass convention, substrate grade, paint system and forming details, and we can respond to the specification you actually need rather than a generic product description.