I have been successful in using the extract sub tool to get a hollowed out model. but...
1. When i do a more complex model I can not get the wall thickness to be thick enough. The wall thickness will also not be uniform. some parts could be .02 and others will be 1.00.
2. It puffs out my model... I loose all the details. Looks like a marshmallow version of the original.
I have been searching for solutions to making inner walls and have found people exporting to meshlab and blender. Does anyone have another solution for zbrush? ... As i am trying to learn those other programs, but am having trouble.