Coil coating is the continuous and highly automated industrial process for efficiently coating coils of metal. Because the metal is treated before it is cut and formed, the entire surface is cleaned and treated, providing tightly-bonded finishes. (Formed parts can have many holes, recessed areas, valleys, and hidden areas that make it difficult to clean and uniformly paint.) Coil coated metal (often called prepainted metal) is often considered more durable and more corrosion-resistant than most post painted metal.
[source: WikiPedia]