My question has always been, okay, you've taken all the paint from the inaccessible areas, how do you now get to these areas and coat them with at least primer?
Well if the area is in accessible chances are there was never any paint there and why rust exists !!
The thing is, rust never sleeps. By getting rid of it in those areas you now have the opportunity to do something to lessen the chance of it coming back. That can be as simple as spraying the insides of frame rails, etc with WD40 or buying and using a product like Eastwood's "rust encapsulator" which comes with a long tube and spray nozzle to access those areas. And chances are the finished car isn't going to see rain, puddles or snow in the rest of its days - thus minimizing rapid progression of any flash rust that does happen to form.
Its like having a cancerous tumor removed, you don't say "that's it" and go home, you get tested to make sure they got it all and then you take steps to make sure it doesn't come back.
