Your vitreous china toilet tank must withstand great water pressure over and over during its lifetime, so it is important to prevent it from developing any cracks. Usually a crack over 1/16 inch wide from the inside of a tank outward will mean it needs to be replaced. However, sometimes the outer surface of a toilet tank will develop hairline cracks in the glazing, which can be repaired. You should make these repairs promptly before water begins to penetrate the glazing.
Step 1 - Disconnect the Water Supply to the Tank and Drain It
Turn off the water supply using the shut-off valve beside the toilet on the wall, and drain the toilet tank by flushing it. Then, use a crescent wrench to disconnect the water supply line from the tank. Remove the tank lid and set it aside on padding, such as towels.
Step 2 - Dry Out the Inside of the Tank Completely
Use additional towels as well as sponges and a chamois cloth to absorb all the water in the tank, both on the tank floor and up the sides. Wipe down the outside of the tank as well, to remove any condensation. The tank must be completely dry inside and out so the waterproof silicone plumbing epoxy will seal properly, preventing expansion of the hairline crack. Once you have mopped up as much water as you can, plug in the portable electric hair dryer and direct its nozzle at the bottom of the tank. Use the highest heat setting to dry the tank inside and out, focusing on the area nearest the hairline crack. Afterward, check that the crack has not yet extended from outside the tank to the inside. If there are no visible cracks on the inside of the tank, continue with the repair.
Step 3 - Apply the Silicone Plumbing Epoxy
Seat the tube of epoxy snugly in a caulking gun, and cut an angled slice off the tip about a quarter of an inch wide. Using firm, steady pressure on the caulking gun trigger, squeeze out a bead of silicone epoxy starting one inch before the crack and continuing another inch beyond it on the opposite end. Spread epoxy over the glazing of the toilet tank using delicate pressure with a putty knife. Avoid widening or lengthening the crack as you smooth out the silicone.
Step 4 - Let the Silicone Epoxy Dry Thoroughly
Allow a period of at least 24 hours for the silicone epoxy to dry thoroughly. Inspect the repair frequently as it dries to watch for other glazing cracks. If it dries without any further visible cracks, you can refill the tank and test your repair.
Step 5 - Refill the Tank
Reconnect the water supply hose to the toilet tank, and restore the water supply by reopening the shut-off valve. Once the tank has refilled, check for any leaks at the repaired spot. If no water is present, flush the toilet once and inspect the caulked spot again. The repair can be considered successful if you still see no dripping around the crack at this point.