The other day I stumbled across this method. I had some steel wool that my wife used in the Kitchen that had a little bleach on it. (I wasn't aware of the bleach at first) I was aging an axe that I had fabricated for a friend. It was basically done and I just needed to go over the last layer of controlled "spit" rust before I soaked it in tongue oil. I rubbed it for about 30 secs and did something else for about 5 min. I came back and noticed all of the lines mocking me as if to say "you can't hide me." The lines were on tig welds that I would consider near perfect. I couldn't believe it. So I took a cap of bleach and put it on a couple old axes I hadn't messed with yet and sure enough. The stress cracks stuck out like a sore thumb. It has to be the most effective and cheapest way to find cracks and past welds. Very good way to talk someone down on the price of an old antique hunk of metal. I wouldn't recommend doing this in an unventilated space as I'm sure it sets off some sort of fume thats not good for ya.
