Things You'll Need
Submersible water pump
4-oz. fiberglass mat
4-inch felt roller
Fiberglass pools are generally durable and long-lasting, but like all other types of pools, they may become damaged over time. One of many good characteristics of fiberglass is that it's easy to repair, and if done properly a patch or repair will make the pool stronger than it was before the damage. Damaged fiberglass must be sanded down and the damaged and removed fiberglass must be built back up with fiberglass material and polyester resin. The supplies are available at your local marine supply store.
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Drain the pool with a submersible water pump to a level where you can work on the crack.
Wipe down the crack and the surrounding 6 inches all the way around the crack thoroughly with a rag and acetone.
Grind directly on the crack, using a die grinder with a grinding cone. Grind the crack until all the damaged and frayed fiberglass is smooth. This will form a groove in the fiberglass. Taper the edges of the groove as far as possible, using the grinder. This will give the repair a maximum amount of surface area to bond to.
Rip pieces of 4-oz. fiberglass mat to fill the groove in the fiberglass. Rip the mat in layers, making each layer a little bit smaller than the last, and the first layer big enough to completely cover the sanded fiberglass.
Fill a small bucket with polyester resin and add 2 percent catalyst, methyl ethyl ketone peroxide, and mix it in with a paint stirrer. Wet out the surface of the repair, using a 4-inch felt roller. Place the first layer of mat onto the repair and saturate the mat with resin, using the felt roller. Roll the mat flat and remove all the trapped air bubbles, using an air roller. Continue the same process until the damaged fiberglass is rebuilt to its original thickness. Let the fiberglass harden for two to three hours.
Sand the repair smooth and level with the surrounding fiberglass, using 400-grit sandpaper on a sanding block. After the repair is completely level with the surrounding fiberglass, re-sand the repair with 1,200-grit sandpaper. Wipe the repair smooth with a rag and acetone.
Pour a small amount of pre-matched marine gel coat into a small bucket and add 2 percent catalyst. Thoroughly mix the catalyst into the gel coat and apply a heavy first coat of paint to the repair with a paintbrush. Let the gel coat harden and apply a second heavier coat. Let the gel coat harden for 24 hours before you fill the pool with water.
Make sure the gel coat is specifically made for pools or marine applications.
Wear a respirator when using the die grinder.