The process of the widely used dye penetrant test method involves coating the test specimen with a visible or fluorescent dye.
The dye penetrant is allowed to sit or dwell in or on the part for a specific amount of time, which ensures that it seeps down into any surface crevice. After the dwell time is up, the excess penetrant solution is removed from the surface.
This can be done with chemicals or water and a developer is applied to draw out the penetrant left behind in surface defects. With fluorescent dyes, ultraviolet light is used to make the bleedout fluoresce brightly, allowing the inspector to easily see the surface imperfections.