The coating is actually very difficult to scratch and is harder than the glass. For example, if you scratch the coating with a coin, it is the metal from the coin you see deposited on the glass surface itself rather than a scratch. For that reason, it is advised that metal objects are kept away from the glass surface as much as possible and certainly no metal objects, especially sharp metal objects such as razor blades, should be used to clean ‘stubborn marks’ on the glass. Instructions on the cleaning can be found here