GB/T 30789.9-2014 "Paints and varnishes - Evaluation of degradation of coatings - Designation of quantity and size of defects, and of intensity of uniform changes in appearance - Part 9: Assessment of degree of filiform corrosion" is applicable to the grading of filiform corrosion defects on coatings, specifying the evaluation principles and classification criteria. Its core lies in grading based on the spread range, density, and depth of filiform corrosion. During assessment, a standard light booth should be used to create a stable observation environment, and a microscope may be employed to assist in measuring the length and depth of corrosion filaments. This provides a unified standard for quantifying the resistance of coatings to filiform corrosion, supporting coating quality control and application selection.