I signed up for this forum so I could use STL2DWG to edit a STL file in Autocad 2011, then convert it back to STL. I spent hours editing a file and to my horror you can't convert back to STL with STLOUT!Basically tried CONVTOSURFACE, didn't work. Tried MESHSMOOTH, didn't work. Neither will create a 3d solid or a "watertight" mesh needed to convert back to STL.Is there anything that is KNOWN to work?

STLOUT works only with solids and watertight meshes.
 
You can try to convert your imported mesh to solid with MESHSMOOTH and CONVTOSOLID - see http://www.cadforum.cz/cadforum_en/qaID.asp?tip=5117 but the process fails quite often.