Swimming pool cracks may occur as a result of shifting in the earth surrounding the pool. In some cases these cracks are cosmetic, but they may also be the source of a leak. Because a crack will grow over time if left unattended, it is important to apply a patch as early as possible. Manufacturers make a variety of products designed for this purpose, some of which will work underwater, while others may require you to first drain the pool. You may need to experiment with several products to find the one that works best with your pool.
Estimate the depth of the pool in the area around the crack. If you suspect that you will need to dive under the surface of the water to perform the repairs, consider wearing swim goggles and a snorkel. Place the dye syringe and epoxy on the edge of the pool nearest the crack. Change into your swim clothes and enter the pool.
Place the end of the dye syringe near where you believe the crack is located. Push down on the end of the syringe to inject the dye into the water. Watch the movement of the dye. If a crack is present, the dye will head toward it. Determine the exact location of the crack you wish to patch.
Apply a small amount of sealant to the crack. Use your hands to work the sealant into the crack, then wipe away any excess sealant with your fingertips or a small metal trowel. Leave the pool and allow the sealant to cure overnight.
Return to the pool and examine the location around the sealed crack. Use a new dye syringe to inject dye around the patch. Watch the movement of the dye to confirm that the patching process was successful. If a crack is still present, the dye will be pulled into the crack, while if the crack is sealed, the dye will just float where you squirted it.
Allow the pool to sit exposed to sunlight for five to seven days. Do not add any chlorine or other chemicals to the pool during this period. Connect one end of a hose to the sump pump, then connect the remaining end to your sewer drain line. Insert the sump pump into the deep end of the pool, then activate the device. Wait for the pump to drain the pool's contents before proceeding.
Locate the site of the crack. Use a clean towel to dry the area around the crack and clear away any foreign material that may be present within the crack.
Fill the length of the crack with the epoxy material. Use your hands or a trowel to push the epoxy into the crack. Dust the surface of the epoxy with silica sand, then level off the surface of the patch with the trowel. Allow the epoxy to cure overnight before proceeding.
Apply a coat of paint over the patched area that matches the color of the pool's interior. Allow the paint to dry, then refill the swimming pool.