ISO 21227-4:2008 "Paints and varnishes - Evaluation of defects on coating surfaces using Optical inspection imagery, Part 4: Evaluation of filamentous corrosion" describes a method for evaluating filamentous corrosion on coating surfaces using digital Optical imagery inspection techniques. Filamentous corrosion is a type of corrosion that develops in filamentous form starting at the edge or damage of the coating. This standard provides a more objective, accurate and repeatability assessment method than traditional artificial visual assessment. It is suitable for filamentous corrosion determination according to ISO 4623-1, ISO 4623-2 or EN 3665 standards.