So i have a 3d model of a girl that looks perfectly good and solid but when i "prepare to print" gaps appear between her skin and her clothes her hair and eyes are hollow.
....why??
It's probably a non-manifold model and/or multiple models in one file. Selecting Separate Connected Surfaces from the Mesh menu should allow it to slice as expected. It will generate multiple meshes in the side bar. Make sure you select Separate Connected Surfaces only after getting the objects positioned on the print bed or, if you do move the object afterward, move all the associated meshes at the same time.