A magnetic flux is sent through the material. At the location of the imperfection a leakage field is created. This attracts metal iron dust, which is sprayed onto the surface. The length of the imperfection can be determined very reliable. Magnetic Particle Testing does not indicate the depth of the imperfection. Acceptance criteria define whether or not the indication is non-acceptable (a defect) or not.
It can be applied to relatively rough and dirty surfaces but flaw sensitivity is decreased for this. Fluorescent particles can be used when maximum sensitivity is required.