Trouble is that you must remove all trace of coatings for it to work, if not then where the coating is intact you get a possibly false negative and where there's remnants of coatings or damaged coatings you get possibly false positives. Also, if you use mechanical methods to remove the coatings such as wire brushes or grit blasting you can peen over any fine cracks which gives false negative results while producing a rough surface that will hold the dye and obscuring any crack indications that are there. I'd stick to close visual inspection with good lighting, a mirror and a magnifier.
