carlosa- thanks but that didn't do the trick, my skull file, while solid has too many interior spaces and they would intersect.
AbnRanger- thanks for the info on Jing! As you can see from my attached screenshot( a cross section for clarity) that I can do as you suggested but the problem is all the time it would take to mine out the interior forms. I tried doing this but it was very labor intensive and I often marred the structure I was trying to get. I only want to make the interior spaces positive- think filling a skull with a material and they removing all the bone. I only want the spaces that once held soft tissue (very halloween!) My idea was to add a greater thickness to the exterior thus giving more room to place my voxel object made from a primitive. This didn't seem to work because whenever I tried to add material to the exterior of the skull it distorted the interior spaces. Another idea was to make a solid voxel primitive that was exactly (or just slightly smaller) than my stl and doing the same "subtract from" with layers but I can't figure out a good way to make it exact. I am thinking that the orifices on the skull (nasal cavity, brain stem opening, etc) are making this even trickier? I could probably go into Rhino and close them if that would help. I don't know if then the "Fill voids" command would work?