What is a substance used as a coating to protect or decorate a surface?

A substance used as a coating to protect or decorate a surface is commonly known as a "finish" or "coating." This can include paints, varnishes, lacquers, and sealants, which serve to enhance the appearance, provide durability, and protect against environmental factors such as moisture, UV rays, and abrasion. These coatings can be applied to various materials, including wood, metal, and plastics, to achieve both aesthetic and functional benefits.