Exterior vs. Interior surface area

Hi,

 

How do I determine the exterior and interior surface area of my model? Is it correct that the exterior surface area is just the full model's surface area from Mass Properties? 

To determine the interior surface area, I selected the 2 curves on the inside of the shelled model but I can't find their surface area. I also tried to undo the shell, so now the model is as it was before hollowing.

I would appreciate if you can clarify how to determine the interior and exterior surface area.

Edu