Hollow representation of a solid?

 From:  Bob (PHOTON713)
Good Morning, all...

I have been generating a number of solids for 3D printing with excellent results from
.stl files from MOI. However, many of the service bureaus quote based on cubic
volume of the solid. In most cases it is not necessary to provide a solid and I could
receive the same results from a hollow pattern. Is there a way to change a solid
to a hollow and define the thickness of the remaining sides? For instance, if I had
a model car that was created as a solid, can I later change it to a hollow with the
sides 1/16" thick and generate a .stl file from the results? I union many individual
solids which is much easier than creating the hollow initially. Thanks