Until recently, most professional fluoride treatments that you received after a dental cleaning were in the form of a foam or gel inside of a squishy tray that fit over your teeth. You wore the tray for a few minutes, then any remaining fluoride was suctioned away, and you were advised to not eat or drink for a half hour or more. This prevented any surface fluoride from being washed or wiped away, allowing the porous tooth enamel to soak in as much as possible.
But with a varnish, the tacky-like texture immediately adheres to the tooth and stays there for several hours. It’s applied with a small brush just one tooth at a time and in most cases, there are no dietary restrictions. Just avoid brushing your teeth until the end of the day, so that the fluoride can be absorbed as much as possible.