After this, inspectors use a developer to draw the solution out of the defects, then use ultraviolet light to reveal imperfections (for fluorescent dyes). For regular dyes, the color shows in the contrast between the penetrant and the developer. This process is ideal for detecting discontinuities open the surface in non-ferrous, and ferrous materials such as aluminum, copper-nickel, steel, or stainless steel. With the different methods of liquid penetrant it can be done on just about anything including welds, castings, machined surfaces, and threaded surfaces. See More