Andrew,
There are three types of polygonal meshes,
surface,surface with skirt, and solid.
The type is selected with a button in the info box.
Please see attached image.
If you seem to have the right mesh type but the mesh
is still hollow then it has at some point "gone hollow",
a condition that cannot be fixed unless you can do enough
"undo" commands to get it back to solid.
Peter Devlin