What is dye penetrant testing?

Dye penetrant process is used to find microscopic cracks in metal.  It is a three part process.  
First, the metal part is degreased with a cleaner/solvent.  A penetrating oily dye is then
sprayed or applied to the area to be tested and allowed to set.  The dye penetrates into the
microscopic crack(s).  The surface is then wiped to remove the extra dye.  A developing white
powder is then sprayed on and allowed to sit.  The penetrating dye leaches back out of the
crack(s) and stains the white powder red.  Voila, there it is, a microscopic crack in what
looked like a healthy Monel propeller shaft, which will inevitably fail.  Dye penetrant testing is
a service provided by Turnstone Marine.