How should scratch resistance be specified for prepainted aluminium?

Start with the scratch risk, not one hardness number

A pencil mark, a stylus penetrating a coating, a polished forming tool rubbing a visible face and two sheets sliding in a stack are different events. One test result cannot represent all of them. The useful specification starts by locating when damage could occur, what contacts the surface, whether the defect is cosmetic or reaches the substrate, and which face is critical.

ECCA's test-method map lists pencil hardness and scratch resistance as separate EN 13523 methods. It also notes that a prepainted finish is already present during forming, so tool condition and contact pressure can turn a processing mark into a finished-product defect. This is why a laboratory value should be tied to the actual handling or forming route rather than used as a standalone marketing grade.

Pencil hardness and stylus scratching answer different questions

EN 13523-4:2014 determines the relative hardness of an organic coating on a metallic substrate using pencils of known hardness. The method can also be used on textured surfaces, but its uncertainty increases as texture becomes more pronounced. ASTM D3363-22 likewise describes a rapid relative ranking and warns that pencil manufacturer, batch and substrate can affect results; for purchase agreements it recommends agreed referee pencils.

BS EN 13523-12:2024 instead addresses resistance to penetration by scratching with a needle. Its scope creates another boundary for thin aluminium: the stylus can deform the substrate rather than simply penetrate the coating, so the report must distinguish coating failure from panel deformation. Neither result automatically predicts every rub, gouge, abrasion or forming mark in service.

Write the acceptance clause as a complete measurement

• Identify the visible face, coating construction, substrate alloy/temper and gauge, specimen source and conditioning.
• Name the current agreed method and version; do not write only ‘pencil hardness’ or ‘scratch resistant’. If parties use another method, define it explicitly.
• Record the tool or pencil set, direction, number and location of readings, endpoint definition and whether the substrate bent or the coating was penetrated.
• State the acceptance limit, repeat or referee procedure, and what happens when visual judgement conflicts with the numerical or ranked result.
• For process-related marks, approve a same-construction first piece and retain photographs, tool settings and the sample; re-review when the coating, substrate, film, tool or route changes.

Qihao perspective: protect the finished face as a process input

From a color-coated aluminium manufacturing and process-communication perspective, ‘2H hardness required’ is incomplete when the actual concern is roll marks, stacking rub or a sliding guide. The same result can sit beside very different tool surfaces, clearances, protection films and handling routes. Naming the damage event usually prevents more disagreement than raising an isolated hardness grade.

Qihao recommends sending the part or profile drawing, visible face, coating and colour, gauge, downstream route, contact points, packaging plan and the defect that must be prevented. That lets the material check and representative process trial be reviewed together before batch release.

This is a marked Qihao perspective based on public standards information and general manufacturing practice. It does not promise a universal hardness grade, scratch load or damage-free result for a specific order; the confirmed specification and representative approval remain controlling.