Liner or chemically resistant layer is the internal layer of the pipe. It is in direct contact with the conveyed fluid. This layer has the function to guarantee the resistance to the chemical corrosion and the impermeability to the pipe. The liner is made of two monolithic sub layers. The inner one, in direct contact with the fluid, is reinforced with glass veil, with a resin content of 90% and the outer one is reinforced with CSM glass, with a resin content of 70%by weight. The standard liner thickness is about 0.5 to 1.2 mm, higher thickness can be produced on request or as per design requirement.

Glass reinforced layers guarantee the mechanical resistance of the whole pipe against stresses due to internal and external pressure, external loads and thermal loads. For GRP pipes, this layer is obtained by applying on the previous partly cured liner, continuous roving of glass wetted with resin, under controlled tensioning. For GRE pipes, the structural wall is wound directly on a wet liner. This layer can contain aggregates like silica sand in order to increase the stiffness of the whole pipe. Thickness of the mechanical resistant layer depends on the design condition.

Top coat or external liner is the outer layer of the pipe which consists of pure resin added with UV protectors to protect the pipe from sun exposure. In case of severe exposure condition like aggressive soils or very corrosive environment, the external liner can be reinforced with a surfacing veil or added with fillers or pigments.