Many of the figure sculptors use Magic Sculpt or Aves.
There is usually a wire and foil skeleton that the epoxy is sculpted over.
Are you sculpting a full head or just modifying one?
You can get large tugs of epoxy putty's if you're adding to a sculpt.
If you are doing a full head to make a mold of most pro's use WED clay which is water based and you don't want it to dry out or others use a non sulfur oil based clay like Chavant. (sp)