It looks like you have many polys not terminating properly in your model. Don't know if that's part of the original model or an artifact of the OBJ import. If you can, use the "select naked edges" script
http://www.kuto.ch/moi3d-shortcuts/
to find the problem areas when trying to create a solid. This will show you what else needs to be 'sewn up' in order for the mesh to be a solid. See below for 'naked edges'