Drag a sharp knife across an area on the plaster. Insert a chisel into the mark you created and chip at it with a hammer until parts of the plaster begin to loosen.
Insert a pry bar between the plaster and the wood. Lift and remove as much plaster as you can using the pry bar.
Use the knife to create another opening in the wall. Insert the chisel into the opening and tap at it with the hammer until it begins to chip away. Use the pry bar again to remove as much plaster as possible.
Scrape away any remaining plaster using a putty knife. Sand the wood using fine-grit sandpaper to remove any grime or debris the plaster left behind.
Apply a coat of varnish to the walls to remove the dullness resulting from sanding.