There are three dye-pene types. The cheapest one is likely good enough ;1- A dye is applied ( usually red), 2- allow to soak ,3 - clean off surface dye( May use a cleaner for this ), 4- spray on a white developer: Depending on experience ,very small cracks can be found in any material. The second type of dye-pene uses a fluorescent dye and UV light ; faster than the red dye but requires a dark room and UV light. The third type uses magnetic fluorescent dye , a magnet and UV light but only works with steel . Cracks revealed by the red dye viewed days later. Also , I think it is the best method to reveal porosity or other non-linear imperfections . Also best if results need to be viewed by several people to make a judgement. Not likely to be your problem , but I remember well several people tying to squeeze under the curtain to view the UV light findings.  The red dye only requires three aerosol cans ( cleaner, dye, and developer) but I have no idea what the price is today.