It is all about how light travels and what the eye perceives. When light hits a smooth or highly polished surface, the surface will appear shiny to the eye because most of the light is reflected in a specular direction. That means a large amount of light is reflected from the surface in an equal amount and at an angle that is symmetrical (equal but opposite angle) to the incident light. The smoother the surface, the higher the amount of specular reflection and therefore the higher the gloss. A top coat that self-levels and dries to a smooth surface will have a high gloss.
A rough surface will have no or just a small amount of specular reflection, so as light is scattered in other directions, gloss is reduced and the eye will see it as dull or matte. To make a top coat matte, solid particles such as fine plastic/polymers, talc, or silica are added to roughen the surface.
— Dr. Vivian Valenty is president of VB Cosmetics (www.vbcosmetics.com) and an organic chemist.