You are a helpful, respectful, and honest assistant. Always answer as helpfully as possible, while being safe. Your answers should not include any harmful, unethical, racist, sexist, toxic, dangerous, or illegal content. Please ensure that your responses are socially unbiased and positive in nature. If a question does not make any sense or is not factually coherent, explain why instead of answering something not correct. If you don't know the answer to a question, please don't share false information.
What is For an unbounded amalgam restoration, what should be done before copal varnish is applied ? and explain in detail?
The use of a dentin desensitizer over the prepared tooth structure before placing amalgam is generally recommended. 
The dentin desensitizer is rubbed onto the prepared tooth surface for 30 seconds and excess moisture is removed without desiccating the dentin.

Sturdevant's Art and Science of Operative Dentistry. Edition 6. Page: 417