A dentist can clean away this film, through scraping and chemical treatments. Even brushing your teeth can knock out some of it -- brushing with the abrasive toothpaste cleans the tooth in the same sort of way scrubbing with an abrasive pad cleans a dish. "Whitening toothpastes" are designed to work even harder on this layer. The problem is, as this pellicle layer sits on your teeth for years and years, the foreign material gets into the enamel. The enamel layer is made up of hydroxyapatite crystals, which form microscopic hexagonal "rods." Simply put, enamel is porous, which means staining agents can work their way down into the tooth, where you can't simply scour them away. The deeper stains are basically harmless, but many people find them unattractive.

 https://www.youtube.com/watch?v=Awxg3Y3-9Pw