A coating can be decorative or protective, or both.
What we see and touch in objects is primarily the coating that is on its surface, not what is underneath.
The surface can often be the weakest link in a material, since the effects of exposure from the elements (light, water, abrasion, dirt, weather) are first seen on the surface. The typical damage observed from the elements are rust, cracking and rotting. A coating protects the surface of materials by separating the material, for as long as possible, from contact with the elements. This is no easy task and many types of coatings exist for different materials and conditions.
A coating for a metal surface (eg: on a car) is designed to withstand different conditions than a coating for a flexible PET bottle, for example. Similarly a coating used to protect the underside of a boat must withstand harsher conditions than a coating used for a metal office cabinet.
A coating protects materials and, by doing so, it prolongs their life. Without such protection from a coating, materials like wood and metal would degrade. Wooden window frames would start to rot quickly without a coating, but if a good coating is applied and maintained, the same window frames can last a lifetime. Iron bridges would begin to rust within months if they were not coated, which would weaken the construction significantly. If specialized coatings are used, bridges can last decades.
Coatings can also be used for decoration purposes, to add color, shine or texture to an object.
Coatings have a dual purpose: protection and decoration. Both are usually desired.
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