This method is used to detect surface-breaking flaws such as cracks, laps, seams, porosity and lack of fusion. It is used on non-metallic materials, on non-magnetic metals that therefore cannot be tested with magnetic particles, on metallic materials that are not excessively porous, and on materials that do not react chemically with the testing media. The penetrant is sprayed onto the part and enters cracks and flaws with openings down to about 1 micron by capillary action. Excess penetrant is wiped off the surface. A developer is then applied, which draws the penetrant out of the surface-breaking cracks and flaws and makes them visible.