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Fiberglass Pool Terminology

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The following terms are often used in describing the composite fabrication process



The finished product, which you are making.


The actual item to be duplicated in fiberglass or other composite materials, which is used to construct the mold. The plug can be the actual part or a custom-fabricated shape, made from virtually any type of material.


The item from which the piece will be made. There are two main types of molds, male and female. A male mold is identical to the item being duplicated, and the piece is made over the mold. A female, or cavity, mold is the reverse of the item to be duplicated, and the piece is made inside the mold. The word can also be used to describe the composite fabrication process: Molding a part.


A solid part constructed from a combination of resin and reinforcing fabric. This term can also be used to describe the process of laying up a part: Laminating a part.

Gel Coat (or Surface Coat)

The term gel coat is often used generically to describe any resin-based surface coating, but the term technically applies to polyester-based materials. The term surface coat can be used to describe either epoxy or polyester materials. Surface coats are specially formulated, thickened versions of resins which can be applied to the surface of a fabrication process or piece to serve as a cosmetic and protective coating.

Release Agent

Any of a number of materials applied to the mold surface before part fabrication, in order to aid in the release of the piece from the mold. These could be waxes, oils or specialty release coatings such as PVA.

Flange/Parting Dam

A temporary fixture attached to the plug when building multiple-piece molds. This generally creates a surface for materials to be molded against, perpendicular to the parting plane of symmetry. The flange aids in clamping or bolting the mold sections together, as well as serving as a mounting point during vacuum bagging operations.

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