Fiberglass notes from THE FIBERGLASS MODEL YACHT BUILDERS GUIDE by David L. Mainwaring 1979

This is a guide to assist the model builder in building model sailing boats from kits with fiberglass hulls and associated components.

"FIBERGLASS"

Fiberglass is the common name for glass reinforced plastics, GRP. The strands are produced in a non-woven cloth called matt and in a woven cloth called fiberglass cloth, boat cloth, tooling cloth and woven roving. After the cloth has been saturated with resin and the resin cured (hardened) we have fiberglass. Fiberglass matt and cloth are sold by the yard in small lots by retailers and by the pound by wholesale suppliers. Matt and cloth are designated in thickness by referring to their weights. Matt is weighed by the square foot with 3/4 to 3 oz. matt being most common. Cloth is also weighed but unlike matt it is weighed by the yard. Thus a 9 oz, cloth has the same glass content as 1 oz, matt. Matt and cloth have different handling characteristics and different conformability as well as different impact and strength factors. Working with matt using brush-on resin requires skill and experience to avoid developing one big mess of glass fibers and resin. You will find with a little practice that matt conforms well when worked with a resin-covered surface with a constantly wet brush. A matt called surfacing matt can be used to work the regular matt in place and squeeze out air bubbles and excess resin. This matt is hard to obtain other than from wholesalers. A layer of cloth works well for surfacing if you can stand the added weight. Fiberglass cloth put up in tape form is widely available. It is a convenient way of getting high quality cloth in small quantities. It is important for the glass resin combination to have as high a glass content as possible for strength. Fiberglass cloths used in boat building must have a special treatment called Chroming to make the glass compatible with the resin. Do not use industrial or auto grade fiberglass cloth unless it is treated.
Fiberglass (GRP) laminates will absorb water through the capillary action of the glass fibers. The problem of water absorption and surface abrasion is overcome in the molding stage by adding a specially formulated resin known as gelcoat. Although gel-coat is available clear, it is usually pigmented to give the surface resin a molded-in color. Polyesters and lay up resins can be pigmented giving the part a solid color throughout. This is an inexpensive way of molding color in but it presents two problems - hidden air bubbles within the laminate and exposing glass strands.