What are common causes of acid taste in mouth and constant coating on tongue?

Many. Acid reflux, gerd, poor oral hygiene are just a few of the possibilities. If you have any gastrointestinal problems see your physician. For your dental problems, see a dentist. Brush and floss after every meal and spend at least 2 minutes each time. Use listerine or prohealth mouth rinse and also get a tongue scraper which should be used once a day.