Oct 07, 2026 · 11 min read

PPGL Galvalume Substrate Paint Adhesion Requirements

Paint adhesion on PPGL is not automatically weaker than on PPGI, but it is less forgiving. The Al-Zn (55% aluminium, 43.4% zinc, 1.6% silicon by typical nominal composition) surface behaves differently from a pure zinc coating: it forms a thin, stable aluminium oxide that resists the very chemical bonds a paint film needs, and it dissolves differently during pretreatment. The result is that a PPGL line must use a pretreatment and primer chemistry designed for Al-Zn, not a galvanized recipe with a different label. When you receive a coil, verify three things: the mill test certificate (MTC) states the substrate as Al-Zn coated steel with a coating mass designation, the paint system is declared as compatible with Al-Zn, and your own sample tests — bend, impact, cross-hatch, and where relevant, reverse impact after forming — pass at the film thickness and bake conditions actually supplied. If any of those three is missing or vague, treat the coil as unverified regardless of how good the surface looks.

Why Al-Zn Is Not Just “Galvanized With Aluminium In It”

Galvanized (GI) steel carries a zinc coating that is electrochemically active and chemically reactive. Zinc oxides and hydroxides form readily, and conventional alkaline cleaning plus a zinc phosphate or chromate-free conversion coating bonds to them in a predictable way. Al-Zn is different in three practical respects.

1. The surface is aluminium-rich at the top

During solidification of the Al-Zn coating, the outer layer is enriched in aluminium. Aluminium oxide is chemically stable, electrically insulating, and thin — often only a few nanometres. That stability is exactly what makes Al-Zn corrosion-resistant in many environments, and exactly what makes it hard to paint. A pretreatment that merely “cleans” the surface without activating or converting that oxide leaves a weak boundary layer between metal and primer.

2. The coating is two-phase

Al-Zn is not a homogeneous alloy. It consists of an aluminium-rich dendritic phase with zinc-rich inter-dendritic regions. Different phases respond at different rates to the same cleaning and conversion chemistry. An over-aggressive etch can preferentially attack one phase and leave a pitted, uneven surface; an under-aggressive one leaves oxide and rolling residue intact. The pretreatment window is therefore narrower than on GI.

3. Zinc and aluminium respond differently to pH

Aluminium dissolves readily in strongly alkaline or strongly acidic solutions; zinc is amphoteric and also dissolves at both extremes but with different kinetics. A cleaner or conversion bath tuned for zinc can over-dissolve the aluminium phase, produce smut, or fail to build a uniform conversion layer. This is the single most common root cause of poor PPGL adhesion that appears only after forming or after weathering.

Pretreatment Chemistry: What Changes for Al-Zn

The following describes general industry practice. Your actual line chemistry, bath concentrations, temperatures and dwell times are contract- and line-specific and must be confirmed with your coater and chemical supplier.

Cleaning: Mildly alkaline cleaners with controlled free alkalinity and good rinsing are typical. Avoid prolonged high-causticity cleaning that attacks the aluminium phase.
Conversion coating: Chromate-free systems based on titanium/zirconium or similar complex-fluoride chemistry are widely used for Al-Zn because they can bond to both aluminium and zinc oxide surfaces. Where a chrome-containing pretreatment is permitted by the applicable regulations and the buyer’s specification, it has historically been used, but many markets now restrict it. Confirm what is actually permitted for your destination and application.
Rinsing: Rinse water quality matters more on Al-Zn. Carry-over of dissolved aluminium and zinc into the conversion bath shifts its chemistry faster than on a GI line, so conductivity and bath analysis intervals typically need to be tighter.
Drying: Incomplete drying before primer application traps moisture and salts at the interface — a defect that often only shows up as blistering or adhesion loss after a bend or after months outdoors.

Primer Chemistry: Why a GI Primer May Not Be the Right Choice

Primers for prepainted metal are formulated around the substrate they must bond to. For Al-Zn, formulators typically select binder systems and pigments that wet and bond to a mixed aluminium/zinc oxide-conversion surface, and that remain flexible enough to survive roll forming, bending and profiling without cracking at the interface.

Key variables a buyer should see declared:

Primer type and dry film thickness (DFT): Typical practice uses a dedicated primer at a specified DFT, with the topcoat chosen for the exposure environment. Thickness must be stated as a range with a tolerance, not a single number.
Bake conditions: Peak metal temperature (PMT) and dwell time. Under-baking leaves the primer under-cured and prone to adhesion loss and blocking; over-baking makes it brittle and prone to cracking on forming.
Backer coat: For many roofing and cladding applications a backer is applied. Its type and thickness should be declared, because it affects coil winding, forming behaviour and cut-edge appearance.
Topcoat system: Polyester, silicon-modified polyester, polyurethane, PVDF and other systems behave differently in UV, humidity and chemical exposure. The correct choice depends on environment and application, not on a generic “best” ranking.

Specification and Verification Table

Use this as a discussion framework. Values must come from your contract, the applicable standard edition, and your coater’s written process specification.

Item	What to verify	Typical form of evidence
Substrate	Al-Zn coated steel declared explicitly; coating mass designation (e.g. AZ designation) and steel grade	Mill test certificate from the substrate producer
Metallic coating mass	Nominal mass per unit area, both sides, with tolerance	MTC; independent test if disputed
Pretreatment	Chemistry family (e.g. Cr-free Ti/Zr type), application method, bath control parameters	Coater process specification / declaration
Primer	Type, DFT range, PMT and dwell	Coater paint specification; line records
Topcoat	Resin family, colour, gloss, DFT range, PMT	Coater paint specification; colour approval
Backer	Type and DFT, if applied	Coater paint specification
Adhesion tests	Test method, result, and whether tested before or after forming	Coater test report; your own sample test
Formability	Bend/T-bend, impact, and any profiling requirement	Sample test on the actual supplied coil
Standard edition	Which edition and which clause values apply	Contract and purchase order

What to Check in the Mill Test Certificate

An MTC for a PPGL coil should let you trace the metallic-coated substrate and the paint system. Read it critically.

Substrate identity. It should name Al-Zn coated steel and give the coating designation, not merely “galvalume-type” or “AZ” without a mass figure.
Coating mass per side. Total coating mass is not the same as per-side mass. Confirm which is reported and whether the figure is minimum, nominal or typical.
Steel grade and mechanical properties. Yield, tensile and elongation matter because forming strain is what often triggers adhesion failure.
Paint system description. Resin family, colour reference, gloss, DFT for primer and topcoat, and PMT.
Test results with methods. “Adhesion: good” is not a result. You want the method, the specimen condition, and a pass/fail or a rating.
Traceability. Coil number, heat number, and date of painting so you can correlate with your own tests.

If the certificate omits the primer type or the PMT, ask. Those two variables explain a large share of field adhesion complaints.

Sample Tests You Should Run Before Accepting a Coil

General industry guidance is that adhesion should be assessed both on the flat and after forming, because a coating can pass on a flat panel and fail at a tight bend. Confirm which test methods your contract requires and which edition of the relevant standard applies.

Cross-hatch / tape test: A quick indicator of interfacial bonding on the flat surface. Useful as a screening test, limited as a predictor of forming performance.
Bend test: Bend the sample over a mandrel or to a defined T-bend and examine for cracking, flaking or delamination. Test at the tightest radius your actual profiling will impose, with a margin.
Impact test: Forward and, where specified, reverse impact to reveal brittle primer or over-baked film.
Solvent rub / cure check: Indicates degree of cure. Over-cure and under-cure both cause problems; the test helps separate them.
Thermal or humidity exposure: Where the application is demanding, an accelerated exposure followed by an adhesion re-test is more informative than a fresh-sample test.
Cut-edge and scribe behaviour: Relevant to how the finished product will perform at sheared edges and fixings.

Record the conditions: film thickness of the tested sample, bake condition, test temperature, and who performed the test. A result without those details cannot be compared with a later batch.

Common Mistakes Buyers Make With PPGL

Specifying “same paint system as our PPGI.” The substrate changed; the pretreatment and primer may need to change with it.
Accepting “adhesion: pass” without a method. This makes disputes unresolvable.
Testing only on flat panels. Most real failures appear after bending or profiling.
Ignoring bake condition. A correct paint chemistry applied at the wrong PMT can still fail.
Comparing coating mass figures that are measured differently. Total versus per-side, minimum versus nominal, and single-spot versus averaged results are not interchangeable.
Assuming a thicker film is always better. Excess thickness can reduce flexibility and increase cracking risk on tight bends.
Treating a standard as self-executing. Always confirm the required edition and the exact contract values, because editions differ in test methods and acceptance criteria.
Leaving the destination environment out of the RFQ. Coastal, industrial, high-humidity and high-UV environments push different requirements onto the topcoat.

RFQ Checklist for PPGL Coil

Send these items so the quotation and the technical offer can be compared like for like. For substrate background, see galvalume steel coil; for prepainted coil context, see PPGI coil.

Application and end product (roofing sheet, wall cladding, profiled panel, other).
Destination environment and expected exposure conditions.
Substrate: Al-Zn coated steel, coating mass per side, steel grade, mechanical property requirements.
Coil dimensions: thickness, width, inner diameter, coil weight range.
Paint system: primer type and DFT, topcoat resin family, colour reference, gloss target, backer requirement.
Required test methods and acceptance criteria, with the standard edition named.
Forming requirements: minimum bend radius, profiling operation, any post-forming processes.
Documentation required: MTC content, test reports, and whether third-party inspection is required.
Quantity, delivery schedule, packing and marking requirements.
Whether a pre-production sample for your own bend and adhesion testing is required before shipment.

Cost Drivers and Quotation Practice

PPGL price varies with the metallic coating mass, steel grade and thickness, primer and topcoat chemistry, colour and gloss, total paint DFT, coil dimensions, quantity, packaging, and the documentation and inspection scope you request. Because these inputs move, a useful quotation is a dated one that states the specification it is based on. A price without a stated substrate coating mass and paint system cannot be compared with another supplier’s price. Request a dated quote against your written specification rather than a headline figure.

Frequently Asked Questions

Is paint adhesion on PPGL weaker than on PPGI?

Not inherently. A properly pretreated and primed Al-Zn substrate can achieve good adhesion. However, the process window is narrower, and a paint system designed for zinc will often underperform on Al-Zn. The practical difference is in how much margin for process error exists, not in a fixed ranking of the two substrates.

Can I use the same primer on GI and Al-Zn?

Some primer systems are formulated for use on both, but you should not assume it. Ask the coater to confirm in writing that the primer and pretreatment are qualified for Al-Zn, and ask for the supporting test basis.

What adhesion test should be in the contract?

At minimum, a specified method for flat-surface adhesion plus a bend or forming-related test at a defined radius. The exact method and acceptance criteria should be named in the contract along with the standard edition. Confirm both with the parties involved before ordering.

Does a thicker paint film improve adhesion?

No. Adhesion is primarily an interfacial property governed by pretreatment, primer chemistry and cure. Excess film thickness can reduce flexibility and increase cracking risk during forming.

How long will a PPGL roof last?

No supplier can responsibly promise a fixed service life. Actual performance depends on the substrate, metallic coating mass, paint system, bake condition, the environment, forming and fixing practice, and ongoing maintenance. Treat any single-number life claim with caution and ask what assumptions it relies on.

What if my MTC does not state the primer type or PMT?

Request it. Those two variables are central to adhesion and cure, and their absence makes a later claim difficult to resolve. If the information cannot be provided, consider that a risk factor in your supplier evaluation.

Next Step

If you are specifying PPGL for a roofing, cladding or profiling application, the fastest way to a comparable offer is to send your written specification — substrate coating mass, steel grade, paint system, DFT, forming requirements and required test methods — and ask for a dated quotation against it. You can reach us through our contact page, and we will respond to the specification you provide rather than to a generic enquiry.