Originally Posted by d2jp
FYI - Prussian Blue by Permatex isn't available in NAPA, Grainer, or any parts store in my area. Even called the machine shops and engine builders and no luck. I used a large Sharpie and it worked great.
A mechanical piping or component supplier for valves in particular will have a blue check dye. We use it in the nuclear industry to blue-check valve seats to ensure no leakage on a closed / seated valve gate. Same principle..... high or low spots....