Which of the following should be done in case of unbounded amalgam restoration before application of copal varnish?
Sealing the underlying, prepared tooth structure is beneﬁcial for any type of restoration. For an  amalgam restoration, this can be accomplished without actually bonding the amalgam to the tooth.  
Nonbonded amalgams can have a sealed, prepared tooth structure through use of a sealer material  (such as Gluma Desensitizer) or a cured resin adhesive placed before the amalgam insertion.
Applied after removal of smear layer only
Smear layer left intact
Smear layer is modified
Dentin desensitiser is applied