Q. Paint system to protect steelwork from corrosion?
In a typical painting system, there are generally three main layers primer, undercoat and finishing coat. Primer acts as the first coat of painting system and adheres to substrate. It serves to provide a foundation for other coats. The mid-coat which is undercoat is designed to increase the film thickness and hinder the background colour. Furthermore it aids in reduction of permeability by incorporating pigments such as micaceous iron oxide. Lastly the finishing coat contributes to appearance of the painting system such as colour. Occasionally it may be designed to provide extra abrasive resistance. Though in terms of corrosion protection to steelworks, it doesn't add much value.
The major component that serves to inhibit corrosion is the primer since it is in direct contact with steel surface. In general primer is pigmented with inhibitors such as zinc and zinc phosphate that protect the steelwork by sacrificial protection. Originally the primer is porous and products produced by sacrificial protection of zinc fills these voids and primer acts as a barrier.