Cavity varnishes have:

Cavity varnish is a solution of one or more resins which when applied onto the cavity walls, evaporates leaving a thin resin film, that serves as a barrier between the restoration and the dentinal tubules. Varnishes neither possess mechanical strength nor provide thermal insulation because of the thin film thickness. The film thickness ranges from 2 to 400 µm. The solubility of dental varnishes is low; they are virtually insoluble in water. Ref- Manapallil 3rd Ed P:81