Here are other opinions for consideration:
According to West Penn NDT book, "Magnetic Particle Inspection (MT) - Wet & Dry
A non-destructive test method used to detect surface and near-surface discontinuities in ferromagnetic material. A magnetic field is induced in a test piece and iron filings are applied. These filings will collect around anomalies in the magnetic field producing a visible area of possible defects.
Dry - An inspection technique where ferromagnetic (iron based) materials are magnetized and dyed iron filings are broadcasted over the material under test, breaks in the surface (cracks, seams) act as magnetic poles attracting the iron filings outlying the defects for detection.
Wet - In the wet method the same magnetizing principal is utilized, but the particles are dyed with a fluorescent coating and suspended in a liquid. The liquid flows over the part and the particles are attracted as in the dry method and viewed under ultraviolet light, this results in a more sensitive test.
According to Charles Hayes, CHARLES HAYES, International Sales/Support Manager, The Lincoln Electric Co., Cleveland, Ohio. He holds NDT Level III certification from the American Society of Nondestructive Testing and is a member of the AWS D1D Subcommittee on Inspection.
"Magnetic powders may be applied dry or wet. The dry powder method is popular for inspecting heavy weldments, while the wet method is often used in inspecting aircraft components. Dry powder is dusted uniformly over the work with a spray gun, dusting bag or atomizer. The finely divided magnetic particles are coated to increase their mobility and are available in gray, black and red colors to improve visibility. In the wet method, very fine red or black particles are suspended in water or light petroleum distillate. This can be flowed or sprayed on, or the part may be dipped into the liquid. The wet method is more sensitive than the dry method, because it allows the use of finer particles that can detect exceedingly fine defects. Fluorescent powders may be used for further sensitivity and are especially useful for locating discontinuities in corners, keyways, splines and deep holes."