I manufactored aircraft parts from 1978 until I closed my business in 1997. In all thoughs years, I never had a drawing that had a mil spec requiring DPI for magnetic steel parts. Milspecs are required for structural aircraft parts.Dye penetrant is an approved method for ferrous (steel). For the past 20 years I have managed both machines shops and NDT labs in the aerospace industry. Dye penetrant is widely used and approved by both milspec and Boeing standards for steel. For inspection of ferrous components Magnetic Particle Inspection (MPI) is preferred for its subsurface detection capability but dye penetrant as still widely used for some applications.
Could you please cite the milspec for the use of DPI in machined, magnetic, steel parts? It's been awhile since I made aircraft parts so, maybe I'm wrong.
Frank