Use the Join command in GSD. This will join (go figure!) all the patches into one element. You can adjust the accuracy between 0.001mm and 0.1mm (I'd suggest using 0.1mm in your case).
If you're struggling with Join, the alternative is Heal, which does the same job but will also fix minor...
When you try to perform the shell, you should get some annotation showing which surfaces are causing problems. Then you'll need to work on those areas to get them to shell.
Don't expect it to be a quick job, a lot of data translations end up needing quite a lot of resurfacing to get them...
1. Bring your assembly from your bin to your workbench
2. Select 'File' from the upper left menu bar.
3. Select 'Export'
4. Select 'IGES' from the list.
5. Return to the workbench, press MB3 (Mouse button 3) and select 'All' from the popup menu.
6. Select 'Instance' from the next pop up...
