What is the primary purpose of cavity varnish used under an amalgam restoration?

The primary purpose of cavity varnish used under an amalgam restoration is to prevent microleakage. Microleakage refers to the small gaps that can occur between the tooth structure and the restoration material, allowing bacteria, fluids, and debris to seep in. This can lead to secondary caries (new decay) and sensitivity.

Cavity varnishes create a barrier that helps seal these microscopic spaces, thus reducing the risk of leakage and enhancing the longevity of the restoration. This is particularly important given that amalgam restorations do not bond to tooth structure as effectively as some other materials. The varnish adds an additional layer of protection against potential damage from moisture or bacteria.

The other options, while related to dental materials and processes, do not accurately represent the primary function of cavity varnish.