Not Solid, just a "Joined Surface

 From:  BurrMan
5786.10 In reply to 5786.9 
"""""""""""the problem must be on export because the SolidWorks guy is opening the STEP files and getting individual surfaces."""""""""""

If you have a "solid" in MoI and export the STEP file, what happens if you just import the STEP file "back into MoI" right after export?

If you export "IGES (surfaces)" then import to solidworks and attempt to Join, solidworks will have many options to show you problem areas. Or, you can attach your file here so it can be looked at. If it's proprietary or something, a private email to Michael can help to handle that.