I seriously am at my wits end on this.... I have several 3D moulding files that are currently meshes of 30k polylines, so they are complicated and busy files.  I need to be able to slice, dice, and make unions with just part of the mesh to another 3D solid.  I have yet to figure out a way to do this.  I can make the shape a surface and slice everything off that I don't need, but I need to find a way to make the leaves a solid so I can add it to another moulding profile.  I tried the new polyhedral mesh to solid app and it did not work.  I have CAD2010 and currently have 28 days left on trial of 2014.... Thanks!  The file is too big to attach, so please go to this page and download the 3D Cad File for the CM8820....

You might give AUTOMESHER a try.  Here is a link to the Automapki website where you can download a free, fully functional, 5-day trial version.

http://www.automapki.com/products/automesher-application.html

I'm using 2013 at the moment.  Like you I was able to convert the 3D model from faces to surfaces but I was unable to convert from surfaces to a solid using two different methods.

I tried 1) CONVERTOSOLID and 2) SURFSCULPT.  Both found inconsistencies in the model they could not handle.