PPGI Coil Defects: Peeling, Blistering and Color Difference

If your PPGI coil is peeling, blistering or showing color differences, the cause is usually traceable to one of three areas: the substrate and its metallic coating, the pretreatment and primer layer, or the curing profile of the topcoat. Occasionally it is a mismatch between the paint system and how the coil is formed or installed. To file a valid claim, you need to stop using the disputed coil, document the defect with dated photographs, batch and coil IDs, storage conditions and forming records, and compare measured results against the values written in your purchase contract — not against general expectations. This article explains how each defect forms, how to separate supplier responsibility from handling or application causes, and how to build a claim file that a mill or trader can actually act on.

Start With the Contract, Not the Defect

Most PPGI disputes are decided on specification wording. A coil that looks wrong may still be within the agreed tolerance, or a coil that looks acceptable may fail a contractual test. Before diagnosing anything, confirm what your contract actually requires for:

• Substrate: steel grade, thickness tolerance and zinc or zinc-aluminium coating mass (for example, the required coating designation under an agreed standard).
• Paint system: primer type and thickness, topcoat type (polyester, silicon-modified polyester, PVDF, epoxy, polyurethane), back-coat type and thickness, and total dry film thickness (DFT).
• Color and gloss: the agreed color standard, the acceptable color difference (ΔE) and the gloss range at a stated angle.
• Mechanical properties: T-bend adhesion, reverse impact, pencil hardness, and bend or forming requirements for your process.
• Test methods and sampling: which standard, which edition, and where along the coil the samples are taken.

Standards such as those published by ISO, ASTM or GB describe test methods and, in some cases, default requirements. They do not replace your contract. Always confirm the required edition and the exact acceptance values with your supplier and, where relevant, your customer before production begins.

Peeling and Adhesion Loss

Peeling is a failure of the bond between paint layers, or between the primer and the metallic coating. Where it occurs tells you a lot.

Peeling at the primer–metal interface

This points to pretreatment or substrate problems: inadequate cleaning, incomplete conversion coating, contamination from oils or fingerprints, or an inconsistent zinc surface. It can also follow from a primer that was undercured before the topcoat was applied, or from a coil that was stored damp and developed interfacial corrosion before painting.

Peeling between topcoat and primer

Usually a cure or compatibility issue — the primer surface was over-cured (reducing intercoat adhesion), the topcoat solvent attacked the primer, or the two layers came from incompatible systems.

Peeling during forming

If the coating cracks or lifts at bends, corners or roll-formed profiles, the paint system may be too hard or too thick for the bend radius, or the T-bend performance may be below what your forming operation needs. This is a specification issue as much as a quality issue: a coating that passes a flat-panel test can still fail a tight bend.

How to test it

Use the T-bend and adhesion test methods named in your contract, on samples cut from the same coil and position as the defect. Record the bend direction relative to the coil’s rolling direction. Adhesive tape pull tests are useful for screening but are rarely sufficient alone for a formal claim.

Blistering

Blistering — raised, paint-covered pockets — is almost always a moisture or contamination problem. Common root causes:

• Trapped moisture or solvent in the primer or topcoat because the cure profile was too fast or the oven temperature was uneven across the strip width.
• Surface contamination on the substrate: residual rolling oil, cleaning chemical residue, or salts left after inadequate rinsing.
• Wet or humid storage before or after painting, allowing corrosion to begin at coating pores or cut edges.
• Incompatible solvents in a topcoat applied over a primer that was not fully cured.

Distinguish blisters that appear on the flat surface (often paint or pretreatment related) from those concentrated near edges or slits (often moisture ingress at exposed steel). Photograph the distribution, size and density, and note whether blisters appeared before or after forming, storage or installation.

Edge Rust and Cut-Edge Corrosion

Prepainted coil protects the face, not the cut edge. Once a coil is slit or a sheet is cut, bare steel and the metallic coating are exposed, and corrosion can start there. Edge rust is not automatically a coating defect. It becomes a supplier issue when:

• The specified zinc coating mass is not present, so the sacrificial protection at the edge is insufficient for the environment.
• Moisture was trapped in the coil wrap during transit or storage, producing rust that spreads inward from the edge.
• The coil was stored on its side, in damp conditions, or with condensation cycles between day and night temperatures.

Document storage conditions, wrap integrity, time in storage and the environment. If edge rust is widespread across many coils from one production period, that pattern supports a production or packing investigation.

Color Difference (Color Drift)

Color difference is measured as ΔE between a sample and an agreed standard, using a defined illuminant and observer. Complaints usually fall into three patterns:

Coil-to-coil variation

Different paint batches, different line speeds or cure temperatures, or a change in primer can shift color slightly. If your project uses several coils on one visible elevation, specify a maximum ΔE between coils, not only against the master standard.

Within-coil variation (side-to-side or head-to-tail)

Usually caused by uneven oven temperature across the strip, inconsistent film thickness, or coating roll wear. Measure at several points across the width and along the length, and record the positions.

Color change after installation

Some fade or shift is normal over time and depends on the resin system, pigment choice, UV exposure, environment and maintenance. Chalking and fading rates differ significantly between polyester, silicon-modified polyester and PVDF systems. Do not accept a blanket statement that a coating will not change color; ask what the agreed color-retention criteria are, over what period, and under which test method.

Always inspect color under consistent lighting. Metallic and pearlescent finishes are especially sensitive to viewing angle, and visual judgments should be backed by instrumental measurement where the contract allows it.

Gloss Variation and Orange-Peel

Gloss variation is typically a cure or film-thickness effect: undercured paint tends to read lower in gloss, overcured paint can become brittle and shift in appearance, and thickness variation changes reflectance. Measure gloss at the angle stated in the contract (commonly 60°, sometimes 20° or 85°) and record the meter, calibration and measurement geometry.

Orange-peel is a surface texture that resembles citrus skin. It comes from application and flow: paint viscosity, wet film thickness, substrate roughness, line speed and oven profile all influence how the wet film levels before curing. Mild orange-peel may be within an agreed appearance standard; pronounced orange-peel on a visible architectural panel is usually a legitimate appearance complaint. Because appearance limits are rarely defined in general standards, write a visual acceptance reference or a measured roughness/gloss criterion into the contract.

Practical Diagnostic Table

The table below is general industry guidance for narrowing down a cause. It is not a substitute for your contract, your test methods or a joint inspection.

How to Document a Valid Claim

A claim succeeds when the other party can verify it without visiting you. Build the file in this order:

1. Stop consumption. Quarantine the affected coils and any formed parts. Do not continue processing, as this weakens the link between defect and supplied material.
2. Identify the material. Record coil number, heat number, paint batch, production date, invoice and packing list references.
3. Record conditions. Storage location, time in storage, wrap condition, humidity or condensation evidence, and handling method.
4. Photograph systematically. Include a scale reference, date, coil ID, and both overview and close-up images. Photograph the defect in consistent lighting.
5. Retain physical samples. Cut samples from the defect area and from an unaffected area of the same coil, label them, and store them dry.
6. Test to the contract method. Use the agreed standard, state the edition, and record instrument calibration.
7. State the claim precisely. Which contractual value was not met, how many coils or metres are affected, and what remedy you are requesting (replacement, credit, or joint inspection).
8. Notify within the contract’s time limit. Many contracts set a notice period after delivery or after discovery. Check yours.

If the dispute involves substrate or metallic coating, the galvanized steel coil specification and its inspection records are relevant evidence. If it involves paint performance, the PPGI coil product data — paint system, DFT, color standard and test results — should be requested alongside the mill certificates.

RFQ and Order Checklist to Prevent Disputes

• Substrate grade, thickness and tolerance, and required zinc or zinc-aluminium coating mass, with the standard and edition named.
• Paint system by layer: primer type and DFT, topcoat type and DFT, back-coat type and DFT, total DFT range.
• Color standard reference (physical master or numeric values) and maximum ΔE against master and between coils.
• Gloss value and measurement angle, with tolerance.
• T-bend, impact, hardness and adhesion requirements matched to your actual forming operation.
• Appearance acceptance criteria for orange-peel and surface defects, ideally with a signed reference sample.
• Test methods, sampling positions and who performs testing.
• Packing, wrap type, desiccant where appropriate, and storage recommendations.
• Claim notification period and the documents required to open a claim.
• Whether third-party pre-shipment inspection is required, and by whom.

Common Mistakes Buyers Make

• Specifying color but not ΔE tolerance, then rejecting coils that are technically within a normal production range.
• Ignoring forming requirements at the RFQ stage, then blaming the coating when a tight bend cracks.
• Storing coils poorly — on damp floors, in condensing conditions, or with damaged wrap — and attributing the resulting edge rust to the mill.
• Processing the whole coil before raising a complaint, which makes it hard to separate material defects from line problems.
• Comparing color under mixed lighting or against a faded reference panel rather than a controlled master.
• Relying on verbal assurances about service life or color retention instead of written performance criteria tied to a test method.
• Missing the contractual notice period and losing leverage even when the defect is genuine.

Where cost is a factor, remember that price is driven by substrate grade and coating mass, paint system type and thickness, color and finish complexity, coil width and weight, test and inspection scope, packing, and order volume. These variables move with raw material markets, so request a dated quotation that states the specification, validity period and payment terms rather than comparing headline numbers from different periods.

FAQs

Is peeling always the supplier’s fault?

No. Peeling can originate in pretreatment, primer cure, topcoat compatibility or substrate contamination — all supplier-side — but it can also follow from over-tight bending, severe forming without suitable coating selection, or mechanical damage during handling. The T-bend result on an unformed sample from the same coil usually separates the two.

How much color difference is acceptable?

There is no universal answer. Acceptability depends on the ΔE value, illuminant and measurement method written into your contract, and on whether the panels will be seen side by side. Agree these values before ordering.

Can edge rust be prevented entirely?

Not on cut edges, because the steel is exposed. You can slow it by specifying adequate metallic coating mass, controlling storage and humidity, minimizing time between cutting and installation, and using appropriate edge treatment where the environment demands it. Actual performance depends on substrate, coating, environment, forming and maintenance.

Does a certificate of conformity prove the coil meets my specification?

It confirms the supplier’s declared results against stated methods. It does not replace incoming inspection or your own verification, particularly for color, gloss and adhesion. Ask which tests were performed, on which samples, and to which standard edition.

What if the supplier and buyer disagree on the cause?

Request a joint inspection or an independent laboratory test using the contract method, with retained samples from both parties. Keep the chain of custody documented. A clear sampling and testing protocol in the contract makes this step faster.

Does a thicker coating always last longer?

Not automatically. Beyond a point, extra thickness can reduce flexibility and worsen cracking at bends, and performance still depends on resin type, pretreatment quality, substrate, environment, forming and maintenance. Match the system to the application rather than maximizing thickness.

Next Step

If you are specifying PPGI for a project, the most useful action is to write the acceptance criteria — substrate, paint system, color tolerance, gloss, adhesion and test methods — into the RFQ before any coil is produced. You can send your draft specification or defect file to our team through the ChinaCNU contact page, and we will respond to the technical points raised. For a quotation, include coil dimensions, substrate and coating requirements, paint system, color reference, test scope and destination so the offer can be dated and specific.

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.