Next select those face normals in wireframe mode that are reversed and flip them. Now your model's geometry table should all have positive coordinates. When you import the object back in to Z, now choose the type of object that you want, surface, solid or etc. The big key here is that you export your model out in positive space. I'm sure there's more than this solution as well, but this one seems to be the most reliable at the moment.
Chances are that the objects contain bad geometry.
Then select all again and stitch them together with the Stitch tool. The Stitch tool will reverse the direction of the face. If deriving the faces and then stitching them back together does not give you the results you are looking for, please send the DWG file for further investigation. The faces that have reversed direction are not stitched with the others and the separate tool can be used to break the object into faces. When this is done the direction can be reversed at the object level.