Many objects, from yacht hulls to toy cars, have an outer shell made of fibreglass. These outer shells are often built up over a form, called a plug. Fibreglass and resin are added in layers, gradually building up the required thickness for the shell. You can purchase all of the materials you will need for this project at a hardware or art store. If you are new to the process, start with a practice shell before you try one that will be used for a project.
Make a plug for the object that you are making a shell of. The plug can be carved out of styrofoam, sculpted out of clay or even built out of wood, so long as it can be disassembled from the inside.
Apply two coats of urethane sealer to the plug if it is made of a porous material such as styrofoam, or a messy material such as clay. Allow each coat to dry completely.
Brush a generous coat of paste wax onto the plug, which will act as a mould release.
Put on rubber gloves and a long-sleeved shirt to protect your skin.
Tear fibreglass mat into 6-inch squares and cut fibreglass cloth into squares of the same size.
Mix a batch of fibreglass gelcoat with its catalyst, following the mixing ratio that is provided on the gelcoat container. This ratio must be accurate for the gelcoat to cure properly.
Brush a layer of catalysed gelcoat onto the plug. This thickened resin product will provide a surface to begin building up the fibreglass.
Allow the gelcoat to harden. The time required will depend on many factors, including the amount of catalyst used as well as the ambient temperature and humidity. It will harden faster in hot, humid weather.
Mix a batch of fibreglass resin with catalyst.
Brush a layer of catalysed resin onto the hardened gelcoat.
Place squares of fibreglass mat onto the plug, overlapping them slightly to ensure complete coverage.
Tap more resin into the fibreglass with the bristles of your brush. This will saturate the resin, turning it clear. Force out any trapped air bubbles as you tap.
Apply a second layer of fibreglass mat, then more resin.
Alternate layers of fibreglass cloth and mat, with resin between each, until you reach the desired thickness.
Apply two final layers of fibreglass mat and resin.
Allow the resin to dry for several hours until it has completely hardened.
Remove the fibreglass shell from the plug. If the plug is made of a softer material, you may have to dig it out in chunks.
Put on goggles and a dust mask.
Trim off any rough edges from the fibreglass with an air saw or rotary tool.
Sand the surface of the fibreglass with 80-grit sandpaper and a palm sander to remove any burrs or other imperfections. Sand it again with 120-grit sandpaper, then a third time with 180-grit sandpaper.
Fibreglass mat has much less texture than cloth. It is used for the outer layers to make sanding easier. Fibreglass cloth is stronger than mat.
Work in a well ventilated area. Fibreglass gelcoat and resin release fumes when they are in their liquid state. Fibreglass resin and catalyst heat up as they cure.