This method is called Liquid penetrant
Here is the procedure:
LIQUID PENETRANT TEST PROCEDURE
SCOPE
Test procedure for inspecting welds and castings for cracks and pores using liquid penetrant.
MATERIAL
Dynaflux, Inc. Cleaner CNF315
Dynaflux, Inc. Penetrant PHF315
Dynaflux, Inc. Developer DNF315
PROCEDURE
1.1. CLEAN
1.1.1. Use cleaner to pre-clean before testing.
1.1.2. Apply a wet film to the part, allowing for adequate solvent time, and wipe clean with cotton clothe.
1.1.3. Repeat until clean.
1.1.4. Allow part to dry.
1.2. APPLY PENETRANT
1.2.1. Spray penetrant onto the area to be examined, wetting the surface well.
1.2.2. Allow 1 to 30 minutes before removing the excess penetrant. (Depend upon the crack size and contamination.
1.2.3. Remove the excessive penetrant, spray cleaner on a clean cloth and wipe surface.
1.3. DEVELOP
1.3.1. Thoroughly shake until the agitator clicks against the bottom and sides of the can.
1.3.2. Spray a thin even coat on the suspected area.
1.3.3. Allow a developing time of 5 to 15 minutes, then inspect. Red lines indicate a crack and red dots indicate mark pores.
Hope this helps, Rich