What each one does. A cavity is acid from mouth bacteria dissolving minerals out of enamel; saliva puts them back; when the dissolving wins, you get a hole. Varnish carries about 22,600 ppm fluoride, a thousand times your toothpaste, and grows a tougher acid-resistant crystal in the enamel ¹. It works on any surface, including exposed roots. Sealants physically lock the deep grooves of back molars shut, where a toothbrush bristle can't reach ². They cover different territory.

The numbers. In kids, varnish cuts new decay by about 43% on permanent teeth ³; sealed molars run 80% fewer cavities at two years ⁴. For children it's settled standard of care, endorsed for everyone under six ⁵. The adult evidence is thinner but points the same way: annual varnish on receding roots dropped new root cavities from 2.6 to 1.4 surfaces over three years ⁶, and adult sealants stay put and protect much as they do in kids ⁷.

For a kid, just show up. Varnish at the cleaning, sealants on the back molars as they come in. The real question is whether you, as an adult, should ask for it. You should if your gums have receded and root surface shows at the gumline, your mouth runs dry (many meds, Sjögren, head/neck radiation), you have active or repeated cavities, or you just came out of braces with white chalky patches.

What you buy. The win is everything that doesn't happen. A sealed six-year molar taken off the table through the high-snacking decade is often the same molar chewed on at seventy. For a high-risk adult on varnish plus prescription paste, the next checkup turns up no new lesion and the crown conversation that was starting doesn't. One filling avoided pays for years of varnish; a root canal, for thirty. Every dollar on school sealant programmes saves up to eleven in later drilling ¹⁰.