Back years ago when I worked in Trailer shop , Would have to find and repair leaks in Tankers and ect, We would use something similar , As I remember it was A purple dye in a spray can, simply spray it on the suspected area and if any cracks the dye would penetrate the crack. Not sure if its the same stuff as the machine shops use. And maybe they don't even use it anymore .But a machinist I used back in the day did.
