It is a fast, economical and widely used non-destructive test method for detecting surface discontinuities in all non-porous materials (metals, plastics or ceramics), also called Liquid Penetrant Control (LPI) or Penetrant Test (PT).

The penetrant test is based on the principles of capillary action where the liquid penetrates the cavity.

Penetrant testing is performed by thoroughly cleaning the test surface, applying colored or fluorescent penetrant, allowing penetration time, removing excess penetrant, and then applying developer (dry or liquid form).

The developer helps pull out penetrant that has penetrated surface discontinuities.

After the developer, the test surface is inspected for leakage under natural light or UV light (depending on the type of penetrant). Fluorescent Penetrant Inspection is the most sensitive test method.