Fiberglass pools are relatively hardy and, with careful use, will last for many years. However, even the most careful people can have an accident and when this happens, cracks can appear in their fiberglass. Instead of having to replace, or at least resurface, the entire pool, a patch to the fiberglass can be installed relatively quickly. Although this will leave a somewhat visible mark, the speed that the pool can be fixed means this is a much cheaper option than a professional repair.
Drain all of the water out of the fiberglass pool. Allow the pool time to dry completely, as the area around the crack needs to be free of moisture for the repair to take hold properly. Examine the pool for cracks and make a note of them all before you begin repair work. Sand down the surface of the fiberglass pool surrounding the crack, to remove loose particles and prepare it for repair.
Cut away the fiberglass around the crack with the diamond-blade cutting tool. This removes any loose fiberglass and other debris, leaving a good surface for the cement to repair the crack. The cut should be 1/2 inch deep when you are finished. Fill the gap with hydraulic cement and allow to set. Paint over the repair with a liquid bonding agent using the paintbrush. Wait an hour for the bonding agent to dry completely.
Mix up the polyester putty according to the instructions provided with it. Apply the putty to the crack and the surrounding area using a putty knife to make the repair as smooth as possible. Wait for the putty to set (times vary, so consult the instructions provided with the putty for details). Sand down the repair so the surface of the fiberglass pool is smooth. Apply the gelcoat using the paint roller.
Cut the fiberglass cloth into strips and apply these over the repair, overlapping the joint between the pool and the repair by 2 inches. Paint the fiberglass cloth with more gelcoat to fix the cloth onto the repair. Apply additional gelcoat using the paint roller, pressing down hard to remove any air bubbles. Allow 24 hours to dry completely. Sand the surface smooth again. Apply a final gelcoat with the paint roller.