The primer coat is used between the material the panel is made from and the layers of paint that will follow. In some cases there can also be filler underneath to make sure the panel is perfectly flat and smooth before starting to paint.
Why use a primer
The colored coat can not be applied on a bare metal panel, the paint would have trouble adhering to the metal and might flake after a short period of time. The primer forms a layer between the panel and the paintwork. The primer will form a strong bond with the panel, but will also make it easy for the colored coat to adhere to. The primer coat forms a layer of protection but can also be used as a type of “emulsifier” between the panel and the paintwork. The primer coat on itself will protect the metal against the elements and prevents oxidation.
What is a primer
The primer coat is the first coat applied to a panel. Although it was mostly used to form a supporting layer between the panel and the paintwork, it is often also applied to plastic panels in order to maintain an overall paint thickness. The primer coat is mostly white or grey in color, but can be colored if needed. The colored coat or uni-coat is applied on top of the primer coat. The primer can be applied on top of bare metal, plastic and even body filler.