250. Whitewashing being often included in the plasterer's specifications, a brief mention of it is desirable. Common whitewash is made by slaking fresh lime, adding enough water to make a thin paste, and is applied to walls, etc. with a brush. Whitewash will adhere best to rough and porous surfaces. For good work, two coats should be put on. By using 20 pounds of sulphate of zinc and 1 pound of salt to each half bushel of lime, the whitewash will be rendered much harder, and will be prevented from cracking. Its durability, especially for outside work, may be increased by mixing a pint of linseed oil with each 2 gallons of whitewash. Whitewash is a very useful agent in preventing decay of wood, and is valuable from a sanitary point of view, in wood, stone, or brick buildings.