The key to a successful liquid penetrant application is precleaning after that the specimen is left to dry for few minutes and after that the liquid is applied to the surface of specimen, and sufficient time is allowed for penetration of surface discontinuities. If the discontinuity is small or narrow. As in a crack or pinhole, Capillary action the penetration, After sufficient time has passed for the penetrant to enter the discontinuity. The surface of the part is cleaned. Capillary action is again employed to act as a blotter to draw penetrant from the discontinuity.

Penetrant testing is successfully applied used on metals such as Alluminum, Magnessium, Brass, Copper, Cast Iron, Stainless Steel, Titanium and most other common alloys, It can also be used to test other materials including glazed Ceramics, Plastics, Molding Rubber, Powdered metal products or Glass.

The major limitation of liquid Penetrant testing is inability to test materials with discontinuities that are not open to the surface or having an porous surface.