It's often hard to shell .stl files if you don't have access to an original solid model file. There are a few programs about that can reconstitute a model with curved surfaces that can be shelled and reconverted into an .stl, but that's often as cack-handed as it sounds and they all cost. TC can do it quite simply from the native .stl, by exploding it down to polylines. While the polylines are still selected, right-click and enter properties from the context menu, go to the "3D" dialogue and give a thickness, which will be the "shell" thickness. There's a quirk here, which is that TC almost always (always in my own experience) has some of the facets/polylines with normals inverted relative to the others, so the thickness of those polylines "shells" outwards. Manually isolate and select those and change the thickness to a negative, that'll correct them. Then reselect all of the facets/polylines together and check the "solid" property, in the property box or selection info. The more facets/polylines there are, the slower this'll be, so patience is a virtue. I've been a road warrior working on an Atom CPU convertible tablet/netbook recently, I'm really getting quite Zen about it. Once they're all solidified, they can be multi-boolean-added together, then the model, your original .stl, now hollow with shelled walls instead of the solid original, can be re-saved/exported as a new .stl. Being hollow, it requires less material to be 3D printed, which makes that process faster.