The foundation of paint - by - numbers is a canvas with numbers. There are pre - marked areas on the canvas that are meant to be filled with paint of certain colors, based on the number designated to each area. Canvases could be made of a variety of materials including cotton, linen or synthetic blends. Canvases with smooth surfaces are preferable since paint can be applied to them more uniformly. While some canvases can be pre - gessoed, guaranteeing a much more feasible surface for the paint while ensuring durability, some need to be scraped before the paint is applied.