The best sustaining topical effect of fluoride is obtained by:

Duraphat, the first fluoride varnish developed in Germany, is a viscous yellowish material, containing 22,600 ppm fluoride as sodium fluoride in a neutral colophonium base (NaF varnish containing 2.26% fluoride in organic lacquer) NaF- 9200 ppm SnF2 - 9500 ppm APF- 12300 ppm Essentials of preventive and community dentistry 5th edition Soben Peter page no. 256