There is a command that is very useful when it comes to assigning thickness to surfaces.
The THICKEN
command
(no alias) allows us to apply a thickness to surfaces, thus creating a 3D solid. The command starts by prompting the surfaces to thicken:
Command: THICKEN Select surfaces to thicken: Selection
We specify the thickness to be applied to all selected surfaces. It can be positive or negative:
Specify thickness <0.0000>: Value