The purpose of form coatings or sealers is some combination of protecting forms, making their absorbency uniform, altering their texture, facilitating form release or making the depth of retardation of concrete uniform. They may be applied to the contact surfaces of forms either during manufacture or in the field. The selection of a type of form coating will depend on what kind of form material is to be protected, what kind of surface characteristics are required in the concrete, how many reuses of the forms are desired, and what demands the environment will place on the physical properties of the coating. In the end, the choice must be based on the manufacturer's reputation, the results of trials, experience and cost. Some panels are coated at the factory. There are two main types: high density overlaid plywood. This kind of form is made with a paper facing on its surface that has first been impregnated with phenol formaldehyde resin and then bonded under high temperature and pressure. The resulting surface completely hides the timber grain of the plywood. It is generally recommended that the surface be lightly wiped with oil or other release agent between uses. Proprietary coatings or treatments are applied to concrete form plywood. Some are epoxy resin formulations that exude oil, thereby eliminating the need for application of a release agent. Others are fiberglass-reinforced plastic coatings. Three kinds of materials are available for the contractor who wants to apply coatings to this own plywood. Solvent-release resins and lacquers are compounds similar to varnish or shellac, or nitrocellulose or acrylic resins and have been widely used for lumber and plywood. With these materials, a release agent is needed to provide a clean release. Catalyzed resin systems are usually a polyester, epoxy or polyurethane resin coating. It is important that the type of epoxy resin chosen be one that cures to a hard surface but also has a certain degree of flexibility. Chlorinated rubber paints have also been used.