The penetrant may be applied to all non-ferrous materials and ferrous materials; although for ferrous components magnetic-particle inspection is often used instead for its subsurface detection capability. LPI is used to detect casting, forging and welding surface defects such as hairline cracks, surface porosity, leaks in new products, and fatigue cracks on in-service components.

The test surface is cleaned to remove any dirt, paint, oil, grease or any loose scale that could either keep penetrant out of a defect, or cause irrelevant or false indications. Cleaning methods may include solvents, alkaline cleaning steps, vapor degreasing, or metal blasting. The end goal of this step is a clean surface where any defects present are open to the surface, dirt, and free of contamination. Note that if metal blasting is used, it may “work over” small discontinuities in the part and an etching bath are recommended as a post-blasting treatment.