Tube inside face transparent.

Hi all.

I developing an aircraft right now, very popular plane, is looking great, but
im having a problem, the inside face of the tube (fuselage) is transparent,
how can i make it look grey or with a texture??:confused: .. i need to do that because
when is going to be time to animate the doors is gonna look transaparent .
I hope some can help me and understand what i mean, i attached a picture
so yo can se whats the problem.