Polyester resin is an oil product that hardness irreversibly when a catalyst and an accelerator is added. If one wishes, polyester resin can be tinted with pigments to create a painting on the surface. In this case, we apply successive coatings of colour. First a test is run on a glass treated with non-stick coating in order to make sure that the desired effect can be obtained. The sample can be detached from the glass and sanded with sandpaper to reach the different layers. For example, we can obtain shading from dark to light or vice versa or shading from one colour to another, whether in a gentle or abrupt gradation.