For an unbounded amalgam restoration, what should be done before copal varnish is applied ?

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