Why is magnesium ribbon cleaned with sandpaper before burning it in air?

Magnesium ribbon is cleaned with a sand paper before burning to remove the layer of magnesium oxide. Magnesium, we know is a very reactive metal. It combines with the oxygen present in the atmosphere to form a layer of magnesium oxide (MgO). This layer interferes in the further reaction of metal. Thus, magnesium is rubbed with sand paper to remove this layer.