You won't have to do it twice if you leave it for 15 minutes the first time. That's what I used to. The main problem is some of the liquid mercury gets caught up in the amalgam matrix. When your brush it out, I number of teensy little mercury beads wind up spraying all over the room.. . . leave it for about 2 or 3 minutes and pour it out, at worst you may have to do it a couple of times. . .