http://www.nps.gov/tps/how-to-preserve/briefs.htm nps.gov Preservation Brief No. 21 "Repairing Historic Plaster Walls and Ceilings" HTML Repairing Historic Flat Plaster - Walls and Ceilings Preservation Brief No. 21 nps.gov HTML nps.gov Repairing Historic Flat Plaster - Walls and Ceilings