Cavity varnish is indicated under amalgam restorations to:

A varnish is an organic copal or resin gum suspended in solutions of ether or chloroform. Varnish is indicated to seal the dentinal tubules, act as barrier to protect the tooth from chemical irritants from cements and reduce microleakage around restorations. Varnish is contraindicated under glass ionomers as they interfere the bonding of tooth to these cements. With restorative resins varnish is not used because the varnish liners dissolve in the monomer of the resin and also they interfere in their polymerization process. Liners are indicated to protect pulp from chemical irritants by sealing ability and stimulate formation of reparative dentin. Commonly used liners are zinc oxide eugenol, calcium hydroxide, flowable composites and glass ionomers. Nisha Garg, Amit Garg. Textbook of Operative Dentistry.  Edition 3. Page:222