Dr. Lekutat's answer:

A coating on your tongue can have quite a number of causes. It can be an accompanying symptom of other disorders, for instance gastritis, infectious diseases or diabetes. Beyond that, it's often a sign of the proliferation of bacteria of the sort that can result from bad oral hygiene. However, overdoing dental care - for example, constantly using anti-bacterial mouthwash, can cause a coating on the tongue. If your tongue remains coated for a long time or you can't scrape off the coating, it makes sense to consult your GP to clarify any further causes.