Oct 05, 2026 · 13 min read

PPGI Blistering and Cratering Defect Analysis

Small blisters, craters and pinholes in the paint film of a prepainted galvanized (PPGI) coil are usually a coating-process problem, not a steel problem — but they are not all the same defect, and they do not all have the same answer. Blisters are raised, closed bubbles under the film; craters are shallow, open depressions with a raised rim; pinholes are tiny holes that penetrate to the substrate. The pattern, size, density and location of each one points to a different root cause: solvent or moisture entrapment, an oven profile that cures the surface before the solvents escape, or contamination on the zinc surface before painting. Whether the coil is still usable depends on whether the defects are cosmetic, whether they breach the film to bare metal, and what the coil will be used for. A coil with isolated blisters on one edge may be perfectly acceptable for a hidden liner panel; a coil with pinholes down to the zinc should be quarantined and assessed against the contract specification before any decision is made.

This article explains how to read each defect pattern, what it usually means in the coating line, and how to structure the conversation with your supplier. It is general industry guidance, not a substitute for the coating system specification, the mill’s process data, or the acceptance criteria written into your purchase order.

Why Blisters, Craters and Pinholes Are Different Defects

Buyers often use these three words interchangeably, which makes the diagnosis harder. In practice they form under different mechanisms.

Blisters

A blister is a closed dome in the cured film. It forms when something volatile — solvent, water, or air — is trapped beneath a skin that has already hardened. The film surface cures first, the volatile cannot escape upward, and pressure pushes the film away from the substrate. Blisters are often soft at first and become brittle as the coil ages. They typically appear in bands, because they follow the oven’s temperature profile across the strip width.

Craters

A crater is an open depression with a raised rim, like a small volcano. It forms when a droplet of contamination — oil, silicone, water, or a foreign particle — sits on the wet film and lowers the surface tension locally. The paint pulls away from that spot and cures around it. Craters are usually small, scattered, and visible immediately after curing. They are a surface-tension phenomenon, not a trapped-gas phenomenon.

Pinholes

A pinhole is a fine hole through the film. It can be caused by a bubble that burst during curing, by a particle that fell out of the film, or by a defect in the substrate surface that the paint could not bridge. Pinholes matter more than blisters because they expose the zinc or the steel, which is where corrosion starts.

Solvent and Moisture Entrapment

The most common cause of blistering in PPGI is incomplete removal of solvent from the primer or topcoat before the film skins over. In a coil coating line the strip moves fast, and the oven has to raise the film temperature through the solvent-evaporation range and then into the cure range in a controlled way. If the early oven zones are too hot, the surface dries before the solvent below it has left. The solvent then boils under the skin and forms blisters.

Moisture behaves similarly. Water can come from humid ambient air, from condensation on the strip, from a wet pretreatment rinse that was not fully dried, or from the paint itself. Moisture-related blisters often appear as a fine, uniform pattern rather than large isolated domes.

Signals that point to solvent or moisture entrapment:

Blisters concentrated in the same position across the coil width, matching an oven zone or a burner position.
Blister size increasing with line speed or with a thicker wet film.
Blistering that appears on the reverse side as well, or on both sides of a coil painted on both faces.
A solvent smell or a soft film when the coil is opened soon after coating.

Oven Profile and Cure Problems

An oven profile problem is not only about peak metal temperature. It is about the shape of the temperature curve along the line.

Too-fast ramp

If the strip heats too quickly in the first zone, the surface of the film cures before the bulk of the solvent has evaporated. This is the classic blister mechanism. The fix is normally a longer or cooler first zone, not a higher peak temperature.

Too-low peak metal temperature

Under-cured film stays soft, retains solvent, and can blister later during forming or in service. Under-cure is often invisible at the coil end and only shows up after the sheet is profiled or exposed to sun.

Too-high peak metal temperature

Over-curing makes the film brittle and can cause craters, cracking and loss of adhesion during bending. It can also drive residual solvent out too violently, producing pinholes where bubbles burst.

Uneven temperature across the strip

Edge-to-edge temperature variation produces defects in bands. If blisters run in a stripe down one side of the coil, suspect an oven imbalance, a burner issue, or an edge that is running hotter or cooler than the centre.

Oven settings are process-specific and depend on the paint system, film thickness, line speed and substrate. Ask the supplier for the cure window recommended by the paint manufacturer and the recorded peak metal temperature for your coil, rather than accepting a general statement that the coil was “properly cured.”

Contamination on the Zinc Surface

Craters and pinholes are more often a contamination story than a solvent story. The zinc surface must be clean, chemically treated and dry before the primer is applied. Anything left on that surface can interfere with wetting and adhesion.

Common contamination sources:

Rolling oil, mill oil or anti-rust oil carried over from the galvanizing or slitting operation.
Fingerprint or handling oils from manual contact.
Dust, zinc fines or oxide particles from the galvanized coil surface or from the line itself.
Silicone or lubricant mist from nearby equipment, which is a notorious cause of craters.
Incomplete rinse after pretreatment, leaving chemical residue.
Condensation on a cold strip entering a warm, humid plant.

Craters from contamination are usually random in position, vary in size, and are often denser on one face if that face was exposed to the contaminant source. Silicone craters tend to be circular with a pronounced rim and appear in clusters. Water or oil droplets tend to leave elongated or irregular craters.

How the Defect Pattern Points to the Root Cause

The table below is a practical diagnostic guide for incoming inspection. It describes general industry patterns; the exact acceptance limits must come from your purchase order and the applicable standard.

Pattern observed	Most likely root cause	Typical location	What to check first
Large, soft blisters in a band across the width	Solvent entrapment from a too-fast oven ramp	Same position on every metre	Oven zone temperatures and line speed records
Fine, uniform blistering over the whole surface	Moisture entrapment or humid ambient air	Both faces, whole coil	Pretreatment drying, plant humidity, paint storage
Small circular craters with raised rims, clustered	Silicone or oil contamination	Random, often one face	Line cleanliness, nearby lubricant mist, roll condition
Irregular craters, sometimes elongated	Water or oil droplets on the wet film	Random, often near edges	Condensation, roller pick-up, coater room conditions
Pinholes penetrating to zinc or steel	Burst bubbles, particles, or substrate defects	Random or along a scratch line	Film thickness, substrate surface, particle sources
Blistering only after forming or profiling	Under-cure or poor intercoat adhesion	Bend and profile radii	Cure records and adhesion test results
Defects concentrated on one edge	Oven imbalance or edge contamination	One edge, full length	Burner condition, edge temperature, edge cleanliness

If you are sourcing prepainted material and want to compare how a supplier describes its PPGI coil coating system and inspection routine, that comparison is worth doing before you place the order rather than after the coil arrives.

Is the Coil Still Usable?

There is no single answer, because usability depends on three things: whether the defect breaches the film, how dense the defects are, and what the end use requires.

Defects that are usually cosmetic only

Isolated blisters or craters that sit within the film and do not expose the zinc are often acceptable for non-visible applications such as concealed purlins, internal liners, or backing sheets. They may still be a contractual non-conformance if the specification sets an appearance standard, so check the order before accepting.

Defects that need a technical assessment

Pinholes, or blisters that have broken open, expose the metallic coating. In a corrosive environment these become initiation points for undercutting, red rust and coating delamination. The rate at which that happens depends on the substrate, the metallic coating weight, the paint system, the environment, forming and maintenance — so no one can promise a specific service life from a photograph.

Defects that justify rejection or rework

Widespread blistering, blistering that appears after forming, or any defect that fails an adhesion or bend test usually indicates a process problem that affects the whole coil, not just the visible area. In that case, sorting the coil is unlikely to recover enough usable material to be economical.

A practical sequence is: photograph the defects with a scale reference, measure their density per unit area, confirm whether they reach the substrate, run a bend and adhesion check on a sample, and compare the results against the acceptance criteria in your contract. Then decide whether to accept with a concession, sort, return, or rework.

RFQ and Inspection Checklist for PPGI Coils

Most blistering disputes trace back to a specification that did not define what was acceptable. Use this checklist when you raise the enquiry.

Substrate and metallic coating: steel grade, thickness and tolerance, zinc coating mass or thickness, surface treatment type.
Paint system: primer and topcoat chemistry, dry film thickness for each layer with a tolerance, and the total system thickness.
Cure requirements: peak metal temperature window and the method used to verify it.
Appearance standard: whether blisters, craters and pinholes are addressed, and at what density or size they become a non-conformance.
Test methods: adhesion, bend, impact, gloss, colour, and any salt-spray or humidity test, with the exact test standard and edition named.
Sampling plan: how many positions per coil, at what frequency, and who performs the inspection.
Documentation: mill test certificate, coating line records, and inspection report format.
Packing and protection: interleaving, edge protection, moisture barrier, and storage guidance.
Handling after delivery: recommended storage conditions and maximum time before forming.

Where a standard such as an ISO, ASTM, EN or GB document is referenced, confirm the required edition and the exact contract values, because limits and test methods change between editions and a general reference is not enough to settle a dispute.

Common Mistakes Buyers Make

Treating all three defects as one problem. Blisters, craters and pinholes have different causes, and a single corrective action will not fix all of them.
Judging from a photograph only. Lighting, magnification and angle change how defects appear. Ask for measured data and a physical sample where possible.
Ignoring the reverse side. Defects on the back face often reveal whether the cause is the oven, the pretreatment, or the environment.
Accepting a coil before checking adhesion. A film that blisters may also fail to adhere, and that shows up later during profiling.
Leaving appearance criteria out of the PO. If the contract does not define acceptable defect density, both sides are negotiating from opinion.
Storing coils badly after delivery. Moisture between wraps and temperature cycling can create new defects that were not present at dispatch.
Assuming a defect will not affect the finished building. Pinholes over a cut edge or a fixing point behave differently from pinholes in the middle of a flat panel.

Frequently Asked Questions

Can blisters appear after the coil has been delivered, even if the coil looked fine at the mill?

Yes. Under-cured or solvent-retaining films can look acceptable at the coil end and blister later, especially after forming, in strong sunlight, or in a warm environment. This is why cure records and a retained sample matter.

Do pinholes always mean the coil will rust?

No. A pinhole that does not reach the zinc, or one in a dry, sheltered application, may cause no visible problem. A pinhole that exposes the metallic coating in a wet or coastal environment is a corrosion initiation point. The outcome depends on the coating system, environment, forming and maintenance.

Can a coil with blisters be reworked?

Sometimes, but it is rarely economic for a full coil. Stripping and recoating is a specialist operation, and the result may not match the original specification. In most cases, sorting or replacement is the more practical route.

Is blistering caused by the steel or the paint?

Usually by the coating process — oven profile, pretreatment, cleanliness or film thickness. Substrate condition can contribute, particularly if the zinc surface has oxide, oil or roughness issues, but the defect itself forms in the paint film.

How should we document a claim?

Record coil number, position, defect type, size, density per unit area, whether the substrate is exposed, and the test results. Include photographs with a scale and, where possible, a retained sample. Compare all of this against the acceptance criteria in the contract.

Does a thicker paint film reduce blistering?

Not necessarily. A thicker wet film holds more solvent and can blister more if the oven profile is not adjusted. Film thickness and cure must be matched to the paint system.

Next Step

If you are diagnosing blistering, cratering or pinholes on a coil, the fastest way forward is to send the defect description, photographs with a scale, the coil specification, and the intended end use. That lets a supplier respond with a specific technical position rather than a general statement.

To discuss a PPGI or GI coil requirement, or to have a defect pattern reviewed against a proposed specification, you can request a quote or technical response. Please include the paint system, film thickness, substrate, metallic coating, end application and any applicable standard with its edition, so the response can be dated and matched to your actual requirement rather than to a generic product description.