To burnish, rub the surface with very fine sandpaper (400 or 600 grit) using a circular motion. Clean the surface with a damp cloth. Create a more polished look by rubbing the surface with the flat side of a clean trowel or putty knife.
The topcoat is usually applied with a trowel soon after the plaster has dried. The topcoat may darken the color a bit. Once dry, you can burnish the surface again with the flat side of a clean trowel or putty knife.
You can also create a good sheen and add protection to the surface by using paste wax for the topcoat. Apply a thin coat of wax with a trowel, and then burnish the surface by rubbing it with the side of the trowel.
Photo © Jahleen Turnbull-Sousa