Can you explain the 'Burnishing' process?

Burnishing isa process by which the surfaces of a planchet or a coin are made to shine through rubbing or polishing. This term is used in two contexts one positive, one negative. In a positive sense, Proof planchets are burnished before they are struck a procedure done originally by rubbing wet sand across the surfaces to impart a mirror like finish. In a negative sense, the surfaces on repaired and altered coins sometimes are burnished by various methods. In some instances, a high-speed drill with some type of wire brush attachment is used to achieve this effect.