Which of the following is not a common type of cavity liner?

Varnish is considered a material used in dental procedures primarily to seal dentinal tubules and provide a barrier to bacteria and moisture, rather than a cavity liner that offers therapeutic benefits. Cavity liners are typically placed in deeper cavities to protect the pulp and promote healing.

In contrast, calcium hydroxide is commonly used as a cavity liner due to its ability to stimulate the formation of secondary dentin and its antibacterial properties. Glass ionomer and resin-modified glass ionomer are also utilized as liners because they provide not only a protective barrier but also release fluoride, which helps to prevent further decay. Therefore, while varnish has its place in dentistry, it does not fulfill the role of a cavity liner like the other materials listed.