In parts of the world with insufficient water and the threat of drought, water storage is a necessity. Tanks that hold thousands of gallons of water can be made from concrete. Sealing these tanks prolongs their lives and reduces the risk of leakage. An underground concrete cistern needs to be sealed before it can be backfilled and buried. Smaller, above-ground concrete water storage systems can be sealed at any time.
Clean all mud, dirt, dust, grime, moss and loose material off of the exterior surface of the concrete water tank with a wire brush.
Apply concrete seam tape to any cracks that are visible on the surface of the concrete, as well as to any joints that you see in the structure of the water tank, for added safety.
Apply water sealer primer to the surface of the tank. Different concrete sealing products are available, so be sure that yours are compatible. Mixing a primer with a sealing coat from a different manufacturer isn't recommended since they may react in unpredictable ways. Depending on the product that you use, it may be either brushed or sprayed onto the surface of the concrete. Allow the primer to dry for 24 hours.
Apply two coats of concrete water sealant to the surface of the water tank using a spray gun. Expect to use about 2 gallons of material for every 100 square feet. This product should be applied thickly, up to about 1/8 inch, to ensure imperviousness to water and leakage.