The primer is the base on which the rest of the coating system is applied.
As a base, it must have strong adhesion to the substrate surface. if the coating system is an inhibitive one, it must contain the inhibitive pigments and be capable of using this pigments in a way which will passivate the metal surface and reduce is tendency to corrode.
In cathodically protective active primer, the coating preferably reacts to a certain extent with the steel surface in order to obtain are even greater and stronger adhesion.
In addition, this primer must react with moisture and electrolytes from an outside source in order to cathodically protect the steel substrate.
Primers are actually the key to the adhesion of the total coating system.
Primers are often applied and allowed to stand for many days or months prior to the application coats.

Primers also, under certain conditions, particularly where they are used for immersion condition or tank lining, must have chemical resistance equivalent to the remainder of the coating system in order to satisfactorily protect against the chemical solution in which it is immersed.
This property is usually associated with the impervious coating system, and unless the primer is as fully resistant as the remainder of the system, the under film breakdown would soon cause coating difficulties with rapid corrosion.