It is used directly in close contact with the composite material during lamination. It must be removed before bonding or painting. Usually woven fibers are used, and the material is nylon, polyester or glass fiber. The fill ply is surface treated with a release agent, but should not be deposited on the laminate. Nylon, polyester, and glass fiber fill plies can be used with most matrix resins, but not with nylon phenolic resins, and are not suitable for use in high temperature molding materials (such as polyimide or bismaleimide). If there is any doubt as to which system the particular filler is used for, check the performance by performing test molding. The standard adhesion test confirms that the release agent coated on the fill ply is delivered to the product during the molding process, and then affects bonding and painting operations. The fill ply accepts the volatile gas between the mold and the thick laminate, which in turn inhibits the formation of pores outside and inside the composite.