Before the EPA banned them, tapping fluids contained Trichlorethylene whose high evaporative rate cooled the tapping action therefor making it easier to tap. They switched to Trichlorethane which was not as good and finally removed the Trichlor entirely. About this time the water soluble crap came in, tried several of these, most were better than nothing. I am now retired, I use some of the old Trichlor containing stuff from a stash of cans obtained at machine tool shows when they gave away cans of the stuff as samples. I've used Do Drill in the past but it really stinks, Rapid Tap is OK too.
