Overview
EN ISO 4628-4:2016 (ISO 4628-4:2016) - Paints and varnishes - Evaluation of degradation of coatings - Part 4: Assessment of degree of cracking - specifies a standardized, visual method for assessing cracking in coated surfaces by comparison with pictorial standards. The method supports objective designation of the quantity, size and depth of cracks using defined rating scales and pictorial examples (Figures 1 and 2) for different cracking patterns (random vs. preferential direction, e.g., wood grain). Illumination for assessment is referenced to ISO 13076.
Key topics and technical requirements
- Rating scales:
- Quantity (density) of cracks: 0 to 5 (0 = none, 5 = dense pattern).
- Size of cracks: 0 to 5, with size thresholds (examples: >0.2 mm visible to the eye; >0.5 mm; >1 mm).
- Depth classification:
- Type a - surface cracks (do not fully penetrate topcoat).
- Type b - cracks penetrate topcoat but underlying coats substantially unaffected.
- Type c - cracks penetrate entire coating system.
- Assessment procedure:
- Compare the test area to pictorial standards for the appropriate cracking type (Figure 1 for isotropic, Figure 2 for preferential direction).
- When mixed sizes are present, report the size rating corresponding to the largest cracks that are typical of the area.
- Assess under controlled illumination per ISO 13076.
- Reporting:
- Express results numerically and concisely (example: “cracking; degree of cracking 2(S3)b”) and include area dimensions or percentage affected.
- Test report minimum content: coating identification, reference to ISO 4628-4, surface details, results, illumination used, anomalies, and date.
Applications and practical value
- Provides a repeatable, visual method to:
- Monitor ageing and weathering effects on coatings (outdoor exposures, accelerated tests).
- Specify acceptance criteria in procurement, quality control and contract inspections.
- Compare performance of paint systems, formulations or substrates (e.g., wood vs. metal).
- Practical tips:
- Use the pictorial guides matching the observed crack orientation.
- Record illumination conditions to ensure repeatability.
- Combine with other ISO 4628 parts for comprehensive degradation assessment (blistering, rusting, flaking, chalking, etc.).
Who uses this standard
- Coating manufacturers and formulators
- Quality assurance and inspection teams
- Test laboratories and certification bodies
- Specifiers, contractors and asset owners evaluating coating condition
Related standards
- ISO 4628-1 (general system and designation)
- ISO 4628-2, -3, -5, -6, -7, -8, -10 (other degradation modes)
- ISO 13076 (lighting and procedure for visual assessments)
EN ISO 4628-4:2016 gives a concise, industry-accepted framework for visually assessing cracking - essential for consistent coating evaluation, specification and lifecycle decision-making.