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Form release agent, also known as concrete form release oil, is sprayed or brushed on molds and forms to prevent concrete from sticking as it cures. The type of form release agent you use depends on the type of mold rubber you’re using as well as the makeup of your concrete admixture. A silicone rubber mold will release concrete easily, but over time it may degrade and lose detail. A urethane rubber mold, however, will bond with a concrete form release agent and stain it.

Suitable commercial form release agents are available for wood, metal and plastic forms. Oils such as straight refined, pale, paraffin-base mineral oil and oil-phased emulsions are satisfactory on wood; a petroleum jelly thinned with kerosene is good for plastic foam forms and plastic form liners. Release agents for steel are typically rust inhibitors. Concrete casters often try to make their own satisfactory release agents, but these concoctions can damage the mold and stain the concrete castings.