Liquid Dye Penetrant Inspection got it's start in the railroad industry.  It was noticed that low viscosity oils would often enter in to crevices to small to be seen with the unaided eye and that the subsequent application of chalk would draw this oil back onto the surface.  This application has grown over the years to be included in many industries and taken on various forms.  The topics suggested by ASNT CP-105 covers the basic principles, theories behind this test method and various applications.  Training can be customized to ensure time is spent discussing the application to be used.