To try and explain what I am doing, we are using SW to create idealised models that can be meshed in our meshing software so that fluid analysis can be run on the models. To do this only the external surfaces can be included in a Parasolid file representing the vessel walls and flow faces. The meshing software get confused if there are gaps between the surfaces, if there are two surfaces in a single space (i.e. overlapping surfaces), if there are surfaces inside the main volume, and if the model is in septate parts (I think this can be explained best by saying that in other words, when the parasolid file is opened in Solidworks it is important there is only a single imported component). 

We also specify boundary conditions on separate parts of the wall. In one project we are trying to do we want to split a pipe so we can specify different properties on different parts of the pipe. We are trying to do this using a surface so a single cylindrical body is represented by 4 surfaces (one for each flat end and two for the curved face). The problem is Solidworks doesn't seem to want to let me do this. Does anyone have any suggestions?