Applying a layer is a process that involves putting a thin covering of material on a surface. This can be done for a variety of reasons, such as to protect the surface from damage or to add a decorative finish. The most common type of COATING is paint, but other materials such as varnish, lacquer, and wax can also be used. The term 'applying a layer' is often used in the context of industrial processes, where it is necessary to apply a protective COATING to a product before it is shipped or sold. The process of applying a layer is usually done with a brush, roller, or spray gun. The COATING in this crossword solution refers to the material that is being applied to the surface.'