Nail polish layer is a term used to describe the thin coating of paint applied to fingernails and toenails. The layer is usually made up of a combination of nitrocellulose, plasticizers, color pigments, and other additives. The most common way to apply the layer is with a brush or a roller. The layer is then sealed with a top COAT to protect it from chipping and fading. The term 'COAT' is often used to refer to the nail polish layer, as it is the outermost layer of the nail.