Question

Why do we rub magnesium ribbion with sand paper before ignity

Answer 1

Because Mg ribbon have MgO(Magnesium oxide). To remove mgo we rub it with sand paper.

Answer 2

we rub it with sandpaper to remove the layer of MgO formed over it's surface formed on reaction with atmospheric oxygen.
When magnesium is burnt , then this MgO acts as a barrier and not let the whole mg burnt with its full efficiency and degrade it's white flame colour.