Magnaflux, along with other companies, still make the penetrant dyes and developers along with cleaners to be used prior to and during the inspection process. There are two basic types, one is a visible light version, the other is a better method but requires the use of UV lighting in a dark booth or room. I have spent many an hour in an inspection booth or room looking at anything from small welded components to relatively large bulkheads hogged out of metal plates or rough forgings.
Long ago and far away, I used to do a quick and dirty bit of penetrant inspection on the footpeg mounts that were prone to cracking and needed to be replaced periodically for a motorcycle I raced at the time. We joking called it "flour fluxing" as a take off on the powdered metal inspection techniques and equipment developed by Magnaflux (hence the inspection term "Magnafluxing"). The mounts would be cleaned and dumped in pan with a very light weight oil with some food dye added to serve as the penetrant oil that was heated. After a couple of minutes the mounts were removed, allowed to cool and recleaned followed by a dusting with very fine flour used as "the developer." If there was a crack, the tinted oil that entered the crack would be drawn out by the flour revealing the crack. Reasonably effective and much cheaper than spending the bucks on the real thing from Magnaflux, Sherwin Williams and such types of outfits.
Now off the thread drift and back to penetrating oils.....