Oct 11, 2026 · 13 min read

PPGI Paint Peeling Causes: From Substrate to Curing Oven

If paint is flaking off your PPGI coil in sheets after profiling, the fastest way to find the fault is to treat the coil, the roll forming line and site handling as three separate suspects and eliminate them one by one. Sheet-like peeling — where the film lifts in large, coherent flakes rather than chalking or wearing thin — almost always points to a failure at an interface: between steel and metallic coating, between metallic coating and pretreatment, between pretreatment and primer, between primer and topcoat, or between the cured film and the deformation it was asked to survive. Your job is to identify which interface failed, because that determines whether the fault sits with the coil supplier, your forming operation, or site storage and installation.

This article traces the coating line backwards — degreasing, passivation, primer cure, topcoat bake, then forming strain — and gives you a practical way to assign responsibility before you raise a claim or change a supplier.

Start With the Failure Mode, Not the Supplier

Before any blame is assigned, characterise the peel. Three questions narrow the field quickly.

Where does the coating separate?

Bare grey steel visible — the failure is at the zinc/paint interface or within the pretreatment layer. This is a coil-line issue in most cases.
Bright zinc visible — the paint has released from an intact metallic coating. Look at pretreatment and primer adhesion.
Paint-to-paint separation — primer and topcoat are splitting. This points to intercoat adhesion, often a cure-window problem.
Coating still bonded but cracked, then flaking — forming strain or over-bake embrittlement is more likely than an adhesion defect.

When does it appear?

Peeling that appears during roll forming, immediately after profiling, or only after the profiled sheet is flexed suggests a ductility or adhesion limit. Peeling that appears weeks later on site, often with white or dark corrosion product underneath, suggests moisture ingress at a damaged edge, cut end or fastener hole.

Is it edge, bend or field?

Edge-only peeling is usually mechanical — slitting burrs, shear damage, or a coating too brittle for the bend radius. Field peeling away from any deformation is much more likely to be a chemical or cure defect from the coating line.

Tracing Backwards: Degreasing and Rinsing Residue

The first place sheet-like peeling originates is the cleaning section. If oils, drawing lubricants or anti-rust films are not fully removed, the pretreatment chemistry cannot reach the steel or zinc surface uniformly, and adhesion becomes patchy. Patches are exactly what produce large flakes rather than fine blistering.

Typical contributors on the coating line include:

Insufficient alkaline cleaner concentration, temperature or contact time for the soil actually present.
Carry-over of cleaner into the rinse stages, leaving a surfactant film that later interferes with passivation.
Inadequate rinse water quality or infrequent rinse-tank dumping, so dissolved salts redeposit on the strip.
Strip speed increases that shorten dwell time without adjusting chemistry — a common cause of intermittent, coil-to-coil variation.

For a buyer, the practical implication is that this class of defect usually shows up as variable performance across a coil or between coils from the same order. If peeling is uniform across an entire coil and reproduces on every coil, cleaning residue is a less likely explanation than a chemistry or cure setpoint.

Chromate Passivation and Pretreatment: The Interface Buyers Overlook

On hot-dip galvanized substrate supplied as GI coil, the surface presented to the paint line is not pure zinc. It carries a passivation layer — commonly chromate-based, though chrome-free alternatives exist — applied to suppress white rust in transit. That layer is a benefit for storage and a complication for painting.

Two failure patterns are worth separating:

Passivation that is too heavy or uneven

An excessive or streaky passivation film can act as a weak boundary layer. The paint adheres to the passivation, and the passivation releases from the zinc. The result is large-area peeling with a characteristic appearance of coating plus a thin discoloured film lifting together.

Passivation that is incompatible with the paint system

Not every pretreatment chemistry is designed to be painted over. If the coil was passivated for storage rather than for subsequent coating, the paint supplier’s recommended pretreatment sequence may not have been followed. This is a specification question, not a shop-floor question: the paint system, the metallic coating and the passivation must be selected as a set.

Where the substrate is supplied specifically for painting, the coating line normally applies its own pretreatment — alkaline clean, rinse, and a conversion coating appropriate to the paint chemistry — rather than relying on a transport passivation layer. Confirm with your supplier which route was used, because it changes how you interpret a peel.

Primer Cure: Under-Cure and Over-Cure Both Fail

Primer is where adhesion is engineered. It is also where the narrowest process window usually sits.

Under-cured primer

If peak metal temperature (PMT) in the primer oven is below the paint supplier’s specified range, the film remains under-crosslinked. It may pass a tape test on a flat panel and still fail in service, because the cohesive strength of the primer is lower than the stress applied during forming or thermal cycling. Under-cured primer often smells faintly of solvent when a freshly opened coil is inspected, and can feel slightly tacky or soft.

Over-cured primer

Excessive PMT or excessive dwell at temperature embrittles the film. A brittle primer survives flat adhesion testing but fractures at a tight bend radius. On a roofing profile with a sharp rib, that fracture becomes the initiation site for flaking.

The key point for buyers: PMT, not oven air temperature, is the control parameter. Line speed, strip gauge, oven load and colour all affect the relationship between oven setpoint and the temperature actually reached at the substrate. A supplier quoting only oven temperatures is not giving you a verifiable cure specification.

Topcoat Over-Bake and Intercoat Adhesion

Topcoat over-bake produces a specific signature: the topcoat and primer separate from each other, or the whole system becomes glassy and cracks before it peels. Over-baked polyester and silicon-modified polyester systems lose flexibility, and the loss is not recoverable.

Over-bake can also be caused indirectly by line conditions rather than by a deliberate setpoint — for example, a line stop with the strip stationary in the oven, or a speed reduction that increases residence time without a corresponding temperature reduction. This is why coil-to-coil variation within a single order can be significant, and why defect investigation should always request the cure records for the specific coil, not a generic process description.

Intercoat adhesion is also affected by the interval between primer and topcoat application. Where the primer is fully cured before topcoat, the topcoat must chemically key to a hard surface. Where the primer is partially cured, the two layers can co-crosslink. Both approaches work; mixing them unintentionally does not.

Forming Strain: When the Coil Is Fine and the Line Is Not

If the coating passes adhesion and flexibility tests on flat, unformed samples but peels after profiling, the coil is probably not the primary cause. Forming-related peeling has recognisable characteristics:

Peeling concentrated at the tightest bend radii, rib corners and hem edges.
Peeling that follows the direction of roll contact.
Cracking visible under low magnification before any flake detaches.
Damage associated with roll condition — scoring, galling, or a worn roll radius that exceeds the coating’s elongation limit.

Coating flexibility is not infinite. The relevant property is the T-bend or bend performance specified for the paint system, and it must be matched to the sharpest radius your profile actually produces — not the radius on the drawing. Where a profile has a very tight rib, a system with higher flexibility may be required even if it costs more or offers slightly different weathering characteristics.

Also check tooling clearance and lubrication. Excessive friction at the roll/sheet interface can generate local heating and mechanical shear that no coating system is designed to survive.

Site Handling, Storage and Cut Edges

Peeling that begins at cut edges, fastener holes, or the underside of sheets in a stack is usually a site or storage issue rather than a coil defect.

Stacked sheets with trapped moisture — water between sheets, particularly in coastal or high-humidity environments, attacks cut edges and can lift coating from the edge inwards.
Incompatible contact materials — some timber, wet insulation and certain sealants can affect coating at contact points.
Mechanical damage during handling — dragging sheets, using damaged slings, or stacking without protection creates edge damage that later propagates.
Cutting method — shearing produces a clean edge; abrasive cutting can leave swarf and heat-affected zones that become corrosion initiation points.

None of these produce large sheet-like flakes on a flat, undamaged panel. If the flakes are in the field, away from any edge or hole, site handling is unlikely to be the cause.

Decision Table: Assigning the Likely Cause

Observation	Most likely stage	What to request or test
Peeling on flat, unformed sample	Coil line: cleaning, pretreatment, primer or cure	Coil cure records, PMT data, adhesion test on retained sample
Peeling only after profiling, worst at tight bends	Forming strain or coating flexibility limit	Bend/T-bend result vs. actual profile radius; roll condition check
Bare grey steel under flake	Zinc/paint interface or pretreatment	Pretreatment route and passivation type confirmation
Bright zinc under flake	Pretreatment or primer adhesion	Primer PMT records; intercoat and adhesion tests
Primer and topcoat separating	Intercoat adhesion / over-bake	Cure window for both ovens; line stop history
Peeling at edges and holes only	Site handling, storage or cutting	Storage practice, cutting method, drainage of stacks
Peeling with corrosion product underneath	Moisture ingress at damage	Handling damage review; edge protection practice

Note that this table is general industry guidance. Your contract may specify particular test methods, acceptance criteria or warranty conditions that override it — always confirm the required edition of any standard and the exact values written into your purchase order.

RFQ Checklist for PPGI Coil

Most peeling disputes trace back to an incomplete specification at the enquiry stage. Include the following when you request a quote for PPGI coil:

Substrate and metallic coating: steel grade, zinc coating mass and type (confirm the required standard edition and exact coating designation in your contract).
Surface condition: whether passivation is applied, and whether it is intended for storage or for subsequent painting.
Paint system: resin family, primer type, topcoat type, nominal dry film thickness for each layer, and colour/tolerance.
Cure specification: required PMT range for primer and topcoat, and willingness to supply cure records per coil.
Mechanical performance: bend or T-bend requirement stated against the actual tightest radius in your profile, plus impact and adhesion test methods.
Coil parameters: width, thickness and tolerance, inner diameter, coil weight, and edge condition.
Protective film: whether removable film is required, and its peel strength and storage life.
Documentation: mill certificate, coating test report, and any third-party inspection arrangement.
Delivery and storage: packaging suitable for the transit duration and climate, and any moisture-barrier requirement.

Cost drivers in PPGI are primarily substrate grade and zinc mass, paint system chemistry, total film thickness, colour and special effects, coil size and tolerance, documentation and inspection scope, and order volume. Because these move with raw material markets, ask for a dated quote and confirm how long it remains valid rather than relying on an indicative figure.

Common Mistakes That Prolong the Investigation

Testing only flat panels. A flat adhesion test can pass while the coating fails at the profile radius. Always test the formed geometry.
Accepting oven air temperature as cure evidence. Only PMT, measured at the substrate, verifies cure.
Changing paint supplier before identifying the failed interface. A new system may fail the same way if the pretreatment or cure window is the real problem.
Discarding the evidence. Keep the failed sheets, the coil identification, the retained sample and the storage conditions. Without traceability, no claim is resolvable.
Assuming a longer warranty implies better performance. Warranty terms are commercial; they do not substitute for a verified cure specification and a matched paint system.
Ignoring the profile drawing. The tightest radius on the finished profile, not the nominal radius, sets the flexibility requirement.

Frequently Asked Questions

How do I tell whether the fault is the coil or my roll forming?

Take a retained, unformed sample from the same coil and run an adhesion and bend test on it. If it passes on the flat but fails at your profile’s tightest radius, forming strain or a flexibility mismatch is the likely cause. If it fails on the flat sample, the coil line is the more likely origin.

Can over-baking in the curing oven cause peeling weeks later?

Yes. Over-bake embrittles the film, and the resulting micro-cracks can allow moisture to reach the interface over time. The initial symptom is often cracking at bends rather than immediate flaking.

Does chromate passivation always need to be removed before painting?

Not necessarily. What matters is whether the passivation chemistry is compatible with the paint system and whether the coating line’s pretreatment sequence was designed for that combination. This must be confirmed as a system, not assumed.

Is peeling at cut edges a coil defect?

Usually not. Cut edges expose bare metal and are the first place corrosion and edge lift begin. Peeling confined to edges and fastener holes normally points to handling, storage or cutting practice rather than to the coating line.

How long should a PPGI coating last?

There is no fixed figure. Actual performance depends on the substrate, the metallic coating mass, the paint system and film thickness, the local environment, the amount of forming the sheet undergoes, and ongoing maintenance. Be cautious of any supplier who offers a specific service life without qualifying all of these variables.

What documentation should I ask for with each coil?

At minimum: mill certificate for the substrate, coating mass result, paint system identification, dry film thickness, and cure records showing PMT. Where the application is critical, agree third-party inspection scope in the contract before production.

Next Step

If you are working through a peeling issue now, gather the failed sheets, the coil identification, your profile drawing with the tightest radius marked, and your storage and handling records. That package is what any serious technical discussion needs.

For specification questions on PPGI coil or GI coil — including substrate, zinc mass, paint system, cure requirements and documentation — send your application details and profile drawing through our contact page. We will respond with the technical questions we need answered before quoting, and a dated quotation based on your actual requirements rather than a generic specification.