Inner panel protection

[Red71gmc]

I am aware of the different methods of out side finishes. What about the important inside finishes for long term protection.
I'm asking about the inner sides of the fenders, cab and bedsides. What do you do to preserve and make a long lasting coating to preserve those parts?
I will be doing a full restore and want to drive the thing so I'm not worried about cost of doing it right. I'm more concerned about doing it right and not destroying all the money spent on the restore to watch it rot from the inside out in time.
What are the best methods to restore a driver all the way? Thank you