Primers
Hi CS
Primers perform several functions. Firstly, it is imperative that they form strong chemical bonds such that when the adhesive (or paint) reacts with it, the interface between the metal and the primer should be as strong as the bonds between the adhesive and the primer. Primers also may contain specific chemicals which assist in minimising corrosion. Primers also enable a prepared surface to be stored for a short time between bonding steps and that helps production scheduling.
Many primers do contain coupling agents and that would improve durability. The secret of success is to apply the primer as soon as possible after the surface has been prepared, because the surface has had the old oxide layer removed (by scotchbrite is one method, but light grit blasting is far more effective) and the surface is chemically active.
Regards
Blakmax