There are many different adhesives that will bond fiberglass, but when taking into account cost, effectiveness, and efficiency, "Plastic Fusion" by the Super Glue Corporation is a smart choice. It is an acrylic-based epoxy, made of resin and hardener, that is designed especially for adhering fiberglass and other plastic-like materials together.
Find a well-ventilated environment in which to work, because it is not safe to breathe the epoxy. The area should be away from any open flames (epoxy is flammable), and should be 68 to 80 degrees Fahrenheit for the epoxy to set quickly and effectively.
Put on rubber gloves and an apron to avoid contact with the epoxy.
Lay a large piece of aluminum foil on a flat surface (at least larger than your piece of fiberglass) to prevent the epoxy from sticking to your work surface.
Lay the piece of fiberglass on the foil.
Dampen a clean cloth with warm water, and wipe the side of the fiberglass that you wish to glue, making sure there is no dirt or grime that will prevent the epoxy from properly sticking. Rub a dry cloth over the newly cleaned fiberglass until you no longer notice any moisture on the fiberglass. Allow a few minutes for the fiberglass to air dry. Repeat this process with the other piece of fiberglass.
Remove the epoxy with the Super Glue syringe and mix the product for approximately one minute.
Apply the appropriate amount of epoxy to the first piece of fiberglass. The ratio is about one dab of epoxy for every square inch of fiberglass you wish to cover. If you need to glue more closely to the edge of the fiberglass, make sure you place the epoxy about one inch in from the edge, so it doesn't squirt out when you press the other fiberglass piece on it. Repeat this procedure for the second piece of fiberglass.
Push the two fiberglass pieces firmly together. You may need to hold the pieces in place until the epoxy initially sets (five to 10 minutes), depending on how you want the fiberglass sheets to be positioned. If you are gluing one piece directly on top of the other, you may not need to hold them.
Wait 24 hours for the epoxy to fully cure.