3D faces to .stl format

[KDA]

It is my understanding that 3D faces (land survey model) can be converted to .stl file format. Using CONVTOSURFACE and then THICKEN to create the 3D object needed to export in .stl creates many 3D objects (thousands) and I cannot use UNION or export in .stl because the 3D objects are more than one.
Can someone tell me where I have gone wrong? My basic input information is a .dwg file with a land terrain model made up of more than 13 000 3D faces, how can this become one 3D object?