This ABRN How-to video Ohio Technical College explains corrosion protection. If the metal is not properly protected, it can rust. On a surface that is going to be painted, the most common products to use are epoxy primer or self etch primer. Once the epoxy or self etch primer is applied, then you can apply 2K primer surface if needed. However, there are some 2K primer surfacers available that are Direct To Metal. This certain primer surface can be applied to direct metal.
Here is a short video discussing the different kind of primers and coating used to protect the backs and inside of parts that are not going to be primed and painted.