Adding material/mass to a mesh I have a blend file that I would like to add mass/material to.  I want to add 4mm of thickness to this hollow model. My goal is to 3D print the model, once it has some volume, so that I can glue the shells together to make a body for a custom action figure. How do I do this? I've attached pictures of my file. Thanks ever so much!
You need to use solidify modifier.

Here is its effect on a cut torus.
A positive thickness value adds a layer inside the model, while negative value adds to the outside. 
Read more at: blender 2.82 Reference Manual