What are the functions of different components in a painting system?

In a typical painting system, there are normally three main layers, primer, undercoat and finishing coat. The primer acts as the first coat of the painting system and adheres to the substrate. It serves to provide a foundation for other coats. The mid-coat, undercoat, is designed to increase the film thickness and hinder the background colour. Moreover, it aids in the reduction of permeability by incorporating pigments like micaceous iron oxide. Finally, the finishing coat contributes to the appearance of the painting system like colour. Sometimes, it may be designed to provide additional abrasive resistance. However, in terms of corrosion protection to steelworks, it does not add much value.

The main component which serves to inhibit corrosion is the primer because it is in direct contact with steel surface. In general the primer is pigmented with inhibitors like zinc and zinc phosphate which protect the steelwork by sacrificial protection. Initially the primer is porous and the products generated by sacrificial protection of zinc fills these voids and the primer acts as a barrier.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.