Liquid Penetrant Testing (PT) 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.  PT is a non-destructive examination method used to locate surface breaking discontinuities in non-porous materials (metals, plastics, or ceramics).  This method may be applied to ferrous and non-ferrous materials, although for ferrous components magnetic-particle inspection is often used instead for its subsurface detection capability.

It is usually a six-stage process:

1. Surface cleaning
2. Application of a penetrant liquid
3. Removal of excess penetrant
4. Application of developer
5. Inspection of test surface
6. Post-inspection cleaning

There are three major groups of penetrant process:

1. Water-soluble
2. Post-emulsifiable with water rinsing
3. Solvent removable, the difference being in the method used to remove the excess penetrant.

In each of these three groups the penetrant solution can contain a dye to make the indication visible under white light, or a fluorescent material which fluoresces under suitable ultraviolet (UV-A) light. The choice of technique for any specific application is based on: the surface finish of the specimen, the compatibility of the materials with the specimen, the sensitivity required, the size, shape and accessibility of the area to be inspected and the ultimate use of the component.