As you probably know when you upload models for printing to Shapeways you need to hollow out your model in order to cut down on costs. I was having a hard time figuring out how to do this in ZBrush when suddenly the answer hit me: Extract a new sub-tool... details below: I used this technique on my chopstick holder that I designed here http://www.shapeways.com/model/77861/chopstick_holder.html In the image below you can see a simple cube that I made. - Using ZBrush masking tools I masked a circle on the surface (made MUCH larger than it needed to be for this example only). - I then reverted the masked to be unmasked and vice versa. -Next I went to the subtools and hit Extract (there are skin thickness options you can explore here also). - I deleted the original tool but you really don't need to do this, I suppose. - As you can see, the model is now hollow... It is my understanding that Shapeways needs at least one hole in the model where unused material can be removed (someone correct me on this if I'm wrong). For my chopstick holder design I made two quite small holes on the bottom. For a design where there is no 'hidden' side I don't know what the option would be. And how small the hole can be I also don't know. Maybe someone from Shapeways can answer that... Hope that helps. Any questions, please ask here or PM me.