Hi Big,
Here is a method to bring your model in. I did this to give you some insight for possible future projects with this customer/vendor.



So first thing to note is I bring it in as a "STEP" file. My guess is you are having to interface with some existing geometry (Like a mould plug). The existing geometry was most likely made in a Parasolids application (And most definitely a "solid body"). If that is the case, ask to use the STEP format. (I have seen many things like this when translating Parasolids, from higher end apps.

With the model, there is a duplicate surface which creates the bad join. Delete the dupe area and recreate the surface and join (If you select the correct dupe to delete, then there is no need to re-create the surface). In the back, there was just a little gap from a fragmented edge. I merged the fragment and recreated the surface to get a solid. No need for the intolerant join.

If it is an "interfacing component", then you would help yourself by discovering the originating app/creater and getting a better export format of the part. They would need to look also at any constraining tolerances, which might be making imports in other apps problematic.

[edited to reflect deleting the correct surface from the duplicated area, removes the need to re-create the surface there]