Why is prepainted aluminium coating peeling? Separate adhesion from cure evidence

Start with the failed interface, not a presumed cause

A useful investigation begins by preserving the loose film, exposed panel and an unaffected control from the same coil. Photograph both sides, mark the rolling direction and record coil, paint, pretreatment, line, position and downstream-process identities. In a multilayer system, separation can occur between coats rather than between the complete coating system and the aluminium; ASTM D3359-23 explicitly notes this limitation.

Use adhesion tests for an agreed minimum, not a universal strength value

ASTM D3359-23 rates relatively ductile coatings on metal using tape over specified cuts. ASTM states that it indicates whether a minimum level is met, is insensitive to fine differences at higher adhesion, and is affected by tape, operator, peel angle, rate, temperature and humidity. A report should therefore name the method, coating thickness, cut spacing, tape, conditioning, operator and failure location rather than provide only a rating.

For coil-coated metal that must deform, the ECCA test map separates rapid deformation, adhesion after indentation and bend cracking into EN 13523-5, -6 and -7. Select the method that represents the actual forming question; a cross-cut result on a flat coupon is not a substitute for the agreed fabrication check.

Use solvent rubbing as cure evidence with stated boundaries

BS EN 13523-11:2019 evaluates the degree of cure of a coil coating through resistance to a specified organic solvent under a specified load. ASTM D5402-19(2024) adds an important boundary: solvent resistance by itself does not indicate full cure, and some coatings become solvent resistant before they are sufficiently cured for service.

Do not invent a universal MEK double-rub limit. Record the coating system, approved solvent, cloth, load, stroke, double-rub count, endpoint, test time and reference or supplier criterion. Follow the solvent safety data sheet and the applicable laboratory controls. If the question is adhesion, pair cure evidence with interface examination and the relevant adhesion or deformation test.

A five-step hold-and-release workflow

• Quarantine the affected material and preserve failed, unaffected and downstream-processed samples with traceable identities.
• Map where separation occurred and record whether the symptom is local, repeating across the width or linked to a forming or exposure step.
• Reconstruct specification, film build, pretreatment, oven/cure, handling, storage and fabrication evidence without changing the suspected samples.
• Run only the agreed tests that distinguish the live hypotheses: adhesion/interface, cure/solvent resistance, deformation, or environmental exposure.
• Release, rework or reject against the purchase specification and representative application evidence; document unresolved boundaries instead of naming an unsupported single cause.

Qihao perspective: make the evidence chain cross the handover point

From a prepainted-aluminium manufacturing and fabrication perspective, the most useful complaint sample is not the cleanest one; it is the one that preserves the failed interface and its process history. Cleaning, scraping or repeating a tape pull before documentation can remove the evidence needed to separate coating manufacture from forming, storage or installation effects.

Qihao recommends agreeing the acceptance method before production and retaining a representative approved panel. For technical review, provide alloy and temper, coating build, sample position, failed and retained film faces, processing sequence, bend/tooling details, exposure history and the exact test method and result.

This is a marked Qihao perspective based on public test-method boundaries and general manufacturing practice. It is not a root-cause finding or a product release decision for any specific order; those require traceable samples, applicable criteria and responsible technical review.