Fiberglass is a very durable material commonly used for water storage tanks. Fiberglass is not typically prone to breaking, cracking or leaking. Once in a while, however, cracks develop. Patching fiberglass is a fairly quick and simple project that requires no special tools or knowledge. If a crack is very large, replacing the tank is probably your best option.
Drain the water from the tank and leave it open to fully dry.
Drill a small hole next to each end of the crack.
Sand the area to be patched so as to rough up the surface of the fiberglass; this allows for better adhesion of the patch.
Cut a piece of the patch material 1 or 2 inches larger than the crack in all directions.
Mix the epoxy thoroughly.
Spread the epoxy with the putty knife over the area that will be covered by the patch. Be sure to jam it into the drill holes at the ends of the crack.
Lay the fiberglass patch material over the area and press it into the epoxy.
Use the paintbrush or putty knife to completely coat and seal the top of the patch area with another layer of epoxy.
Allow the epoxy to cure for 24 hours before using the water tank.