Fibreglass is a lightweight, strong building material that is used in hundreds of products. If you are using fibreglass in a craft project, it is important to know how to use it properly. Fibreglass matting (often simply called "mat") refers to a sheet made from a loose weave of chopped fibreglass strands. Because it is so loosely constructed it can be formed around corners and into sharp angles that other fibreglass products cannot. A great way to learn to use the material before tackling larger projects is to make a simple fibreglass box. If you can master hard angles and corners, you can glass anything.
Paint a thin coat of petroleum jelly onto a cardboard box to act as a release agent.
Rip the fibreglass mat into many 5-inch squares. You will also want to rip several small, loose circles for the corners. There will be some loose fibres which can irritate your skin, so wear rubber gloves and a paper suit.
Mix a batch of fibreglass resin and its catalyst in a disposable bucket according to the instructions on the resin container.
Paint a layer of catalysed resin over the top and sides of the box.
Lay squares of fibreglass mat into the resin, overlapping them slightly. Wrap squares around the side corners for strength. Use the loose circles on the pointed corners (these will be the most challenging part of the box).
Tap more resin into the fibreglass mat to fully saturate it. Tapping (as opposed to brushing) will help to keep the squares from sliding around. It also removes trapped air bubbles, which can weaken the fibreglass.
Add another layer of fibreglass squares onto the box, then saturate them with more resin. Repeat this step several times until you feel you have the hang of it.
Allow the fibreglass to fully cure until it has hardened.
Sand the entire outer surface of the fibreglass box, starting with coarse sandpaper and working to finer and finer grits.
Remove the cardboard box from the fibreglass one. It should pull free easily because of the petroleum jelly.
Trim the rough edges from the bottom of the box with a saw or rotary tool. Wear a dust mask and eye protection for this step, you don't want to get the dust into your eyes or lungs.
The other commonly used type of fibreglass is cloth, which is less messy but is much harder to work around corners.
Resin fumes are toxic. Make sure you have proper ventilation before using.