This type of testing uses the forces of capillary action to detect surface breaking defects. It is impossible to detect defects which do not break the surface but it can be used on both magnetic and non-magnetic materials providing they are non-porous. The penetrant (usually red) is applied to the cleaned surface and left to dry.

The surface penetrant is then removed using a solvent. A developer (usually white) is then applied and left for approx 15 mins. The developer draws out any penetrant remaining in any surface discontinuities with a blotting action.