The best time for typical fluoride application is-

The major cariostatic action of fluoride is now understood to be topical/post-eruptive. After a tooth erupts, fluoride present in saliva, plaque fluid, toothpaste, gels, varnishes, etc. acts directly at the enamel surface. It :

• Inhibits demineralization
• Enhances remineralization of early carious lesions
• Produces a more acid-resistant mineral phase
• Can inhibit bacterial metabolism at sufficient local concentrations

Newly erupted teeth are particularly important because they undergo post-eruptive maturation, but a topical fluoride agent requires an erupted tooth surface on which to act.