Liquid penetrant testing is a traditional non-destructive method for inspection of surface breaking flaws such as cracks and porosity in test objects with non-porous surfaces. The tested surface is cleaned and coated with a penetrant solution either a visible or fluorescent dye that is drawn into the discontinuity by capillary action. The excess solution is removed from the surface, but the penetrant remains in the surface breaking flaws. A white developer is then applied to draw the penetrant out of the defects and shows a contrast image. After the evaluation of the indications the surface is cleaned.

Liquid penetrant testing can be used on relatively dirty surfaces, but the sensitivity is decreased, but a fluorescent penetrant can also be used that gives increased sensitivity. The length of an indication can be accurately determined, but the depth cannot be determined with this method.