Liquid penetrant testing is one of the simplest NDT methods. This method is used to reveal surface discontinuities by bleed out of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a "clean" surface discontinuity by capillary action. After a period of time called the "dwell time", excess surface Penetrant is removed and a developer applied. This acts as a blotter that draws the Penetrant from the discontinuity to reveal its presence.