Why should a magnesium ribbon be cleaned before burning in air?

Magnesium ribbon made of  pure magnesium metal due to its high reactivity  towards oxygen molecule led to the oxidation of magnesium when it come in contact with air this led to formation of layer of oxide on the surface of the magnesium ribbon, this layer acts as a protective layer and prevents further oxidation of pure magnesium to  magnesium oxide. This layer is needed  to be removed to avail pure magnesium which can be easily be removed by sanding the surface of ribbon using a sandpaper 

     
(magnesium) (oxygen)  (magnesium oxide)