Do I need to sand this whole thing down!

I'm currently in the process of getting my 68 dart ready so I can paint it. I've sanded a door, trunk, both front fender panels, some of the engine bay and some of the trunk.

Now I understand sanding down the outside of the car, but what about the inside fender panels, and the floor pans and trunk underside and trunk floor, etc... The kinda not so seen areas. Do I really need to sand down this whole entire car to bare metal?

Is it bad to just rough up the paint and then go over it?