A form release agent must do several jobs: permit clean release of formwork from the hardened concrete during stripping; protect the formwork for long life and extensive reuse; help produce a hard, nonpowdery, stain-free concrete surface with a minimum number of defects; and prevent corrosion of steel forms and consequent staining of the concrete surface. The way a form release agent performs is markedly influenced by the form material. Therefore the release agent should not be chosen until the form material has been selected. For wood forms most of the commercial oils are satisfactory: straight refined, pale, paraffin-base mineral oil and oil-phased emulsion have been successfully used. The oil should be capable of penetrating the wood to some extent while leaving the surface only slightly greasy to the touch with no free oil on it. Form release agents that are satisfactory on wood are not always suitable for steel forms. Release agents for steel forms should contain a rust inhibitor and be free of water. So-called "fiberglass" forms are really plastic forms reinforced with fiberglass. These forms and plastic form liners can be used a few times without a release agent because of their hard, smooth finish but after a few uses the surfaces become so rough as to necessitate the use of a release agent. Most rubber mattings do not require application of release agents if the surface is thoroughly cleaned and moistened with water just before concrete is placed. Some users prefer to coat the rubber with a thin film of castor oil or other vegetable oil, lanolin, or water emulsion wax. Paraffin oil generally is lightly sprayed over plastic foam forms or form liners. Castor oil, or petroleum jelly thinned with kerosene, also are good release agents. Concrete forms, even if they are ground, smoothed and polished, require a release agent to prevent sticking and enable easy stripping. Light colored petroleum oils or oil emulsions or various types have been used successfully. Watertight, oil-treated hardboards produce concrete surfaces of uniform color and require only small amount of a release agent. When plaster waste molds are thoroughly dry, two coats of white shellac should be applied to the mold surface to make it waterproof and nonabsorbent. Before concrete is placed in it, the mold should be lightly greased with a nonstaining soft yellow cup-grease.