ISO 4628-7:2016

Overview

ISO 4628-7:2016 - Paints and varnishes - Assessment of degree of chalking by velvet method - specifies a visual/physical test for chalking (formation of loosely adherent powder) on exterior coatings. The standard is aimed particularly at rough surfaces (surface roughness greater than segment 4 of comparator G per ISO 8503‑1) and is applicable to coatings on mineral substrates such as fibre cement, brick, concrete and renderings. The method is suitable for laboratory and on‑site evaluation by experienced operators.

Key topics and requirements

• Scope and applicability
  • Designed for rating chalking on white or coloured exterior coatings on rough surfaces; can be used on smooth surfaces but ISO 4628‑6 (tape method) is preferable.
  • Applicable to coating systems on mineral substrates regardless of surface structure.
• Principle
  • Loosely adherent powder is removed from the coating surface using a fabric (velvet, black wool felt, velveteen for light coatings; white fabric for dark coatings) and the removed material is assessed against a rating scale.
• Assessment procedure
  • Perform under specified illumination (see ISO 13076) and in duplicate unless otherwise agreed.
  • Press fabric with index finger, rotate 180°, remove and inspect fabric under diffuse light.
  • Mechanical apparatus may be used if results are comparable; a single wipe is permitted.
• Rating scale
  • Numerical ratings 0–5 (derived from ISO 4628‑1): 0 = no perceptible chalking; 1 = very slight; 2 = slight; 3 = moderate; 4 = considerable; 5 = severe.
  • Relative ranking method - not intended as an absolute contractual acceptance test between parties unless further specification is agreed.
• Reporting
  • Test report must identify the coating, reference ISO 4628‑7, surface type and location, numerical rating (mean, range, number of assessments), fabric details, method (finger or apparatus), deviations, anomalies, and date.

Applications and users

• Practical for quality control, product development and condition surveys of exterior coatings where chalking is a concern.
• Typical users: paint manufacturers, coating formulators, surface inspection teams, specifiers, laboratories performing weathering and degradation tests, and building envelope inspectors.
• Recommended when evaluating rough mineral substrates after weathering exposures (natural or accelerated); agreement on test conditions (weathering method and substrate) is required when referenced in specifications.

Related standards

• ISO 4628‑1 - General introduction and designation system (rating definitions)
• ISO 4628‑6 - Chalking assessment by tape method (preferred for smooth surfaces)
• ISO 13076 - Lighting and procedure for visual assessments of coatings
• ISO 8503‑1 - Surface roughness comparators for blast-cleaned surfaces

Keywords: ISO 4628-7, chalking, velvet method, paints and varnishes, coating degradation, chalking assessment, exterior coatings.