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What is The best sustaining topical effect of fluoride is obtained by: and explain in detail?
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