The liquid penetrant inspection method is based on the ability of certain types of liquids to enter into voids and crevices by capillary action and to remain there when the surface liquid is removed. Thus, liquid penetrant inspection, when done properly is a reliable and revealing method for detecting discontinuities open to the surface. Very small and tight imperfections can usually be shown. The several variations of the method each have their advantages and disadvantages.

As-welded surfaces are suitable for penetrant inspection methods if the surface contour does not contain sharp depression between beads or weld ripples that could interfere with complete cleaning and complete excess-penetrant removal. Any one of these could result in false or irrelevant indications. Such weld surfaces should be ground smooth prior to inspection.

-AWS