Andy -
I just had a brain fart - Its Dye penetrant testing.
The stuff is 2 parts - called 'dye penetrant solution' (that's part A, and 'developer' (part B)
Its sold seperatly or as a kit.
You spray on part A, let sit, spray Part B, wipe. The cracks get exposed...
No connection to mag flux or mag particle - my bad
