Smoking a mold, or using a spray mold release, causes the vent lines to be plugged up. It also causes the boolit to be undersized,,--smaller that it was designed to be.
I'd like to ask John Lee why he insists on putting that in his instructions. It's never recommended over on castboolits . com, but a lot of people start out that way because the mold maker says to.
I sent a 6 cav. 9mm 124 RN mold back to lee. It was misaligned. I got back a mold that had been tested, it was already smoked. First thing I did was remove the black junk from the cavities!
The replacement works just fine.