I have created a sphere with a cavity (a recessed area) in Cinema 4D. I then saved it as an .OBJ. I then imported it to ZB 3.1 but the outer of the sphere model looks transparent as I can see the inner cavity from the side view. The sphere is transparent.
Any ideas wht is causing this or if I need to do something else befroe exporting from C4D?
Thanks
Switching on Tool: Display Properties: Double should solve the problem.
Thanks Marcus I will try that! 
but why i have to do double and does it affect the whole arrangement of mesh??
The problem is that your normals are flipped. The best solution is to find the setting in your other app that’s causing this to happen when you export. However, ZBrush’s double setting instructs ZBrush to render both sides of the polygons rather than one. It doesn’t affect the geometry in any way. It simply allows you to see the model like normal without having to track down the specific setting that’s to blame.