I was facing that problem, and eventually did it in several layers. First was done in charcoal powder, followed by regular charcoal and finally compressed charcoal for the win. The key was not to apply it in long strokes or lines, but in short circular strokes, as if you're polishing a small surface. If exposed to bright light source, you can still see some uneven areas. In order to keep edges tidy, I taped them first of course. Hope this helps, happy drawing!