The GRP pipe wall consists of three layers perfectly adherent with one another, each having different characteristics and properties in relation to their function. The properties of chemical resistance and impermeableness are, anyway, equivalent for the three layers which are namely:

• Liner:
  It is in direct contact with the conveyed fluid and guarantees the maximum resistance to the chemical attack from the fluid itself. Moreover, the liner presents an internal surface particularly smooth. The liner is composed of glass veil and chopped glass (glass mat).
• Mechanical resistant layer:
  Its function is to render the pipe wall resistant to the stresses due to the design conditions (stresses due to the internal and/or external pressure, flexural strength due to the external loads). The thickness of the filament depends on the design conditions. The mechanical layer is composed of continuous glass filament roving, chopped glass and filler if necessary.
• Top coat or external layer:
  It has a thickness of about 0.2 mm and consists of pure resin with or without glass reinforcement. It guarantees the complete impregnation of the peripheral fibers, thus yielding the external pipe surface completely free of protruding fibers and well finished.