Question

Sometimes the magnesium is polished to remove any white film on its surface before beginning the experiment?

Answer 1

Yes. The reason for this is that the tarnish is magnesium oxide, so you need to remove it so you have a pure sample of magnesium before burning it to determine the product produced from the burning. If you don't remove the tarnish, the ribbon may not burn readily, and your results will be adversely affected.