Hi Micha, also if you can't find a way in Rhino to break a mesh apart into connected pieces, you can use this utility instead:

http://moi3d.com/download/SeparateOBJ.zip

To use that, export your joined objects from MoI to an OBJ file but make sure to turn the welding option off.

Then run that SeparateOBJ.exe utility on that file, and it will edit the OBJ file to break it into chunks like that.

Then when you load it into Rhino after that, you will have a watertight mesh without cracks but with a separate mesh object for each original surface in the solid.

- Michael