Today's plastic toilet tanks rarely crack, but older porcelain tanks can crack because of damage or impact. It's common to replace a toilet tank when a crack occurs, because repairing the crack can demand more time, effort and cost than a simple replacement. However, small cracks lend themselves to a quick repair job needing only a few easily-obtainable supplies and tools. Give a toilet tank crack repair a chance before looking for a new tank.
Shut off the water to the tank and dry the tank completely. Clean the crack area.
Drill a hole completely through the porcelain with a Dremel tool and diamond-encrusted rotary grinder bit. Drill at the crack's farthest visible extent. Wet the drill bit periodically with a sponge to keep it cool, which will control dust and keep the crack from spreading.
Drill out a V- or U-shaped area along the crack to remove dirt and give the epoxy a large surface for bonding. Drill the area approximately 1/8 inch deep and 1/8 inch wide.
Fill the crack with industrial white epoxy paste. Smooth the epoxy so that it raises just a little higher than the level of the toilet tank surface. Put a piece of clear packing tape on top of the epoxy-filled crack to keep the epoxy in place while drying.
Sand the epoxy down to the tank's surface. Start with the coarsest-grit sandpaper (40- to 60-grit) and slowly graduate to finer-grit sandpapers (600-grit). Continue until the cracked area seems smooth. If air bubbles occur, add more epoxy and re-sand after the epoxy dries.
Spray surface repair refinishing spray on the dried epoxy and allow to dry, followed by a clear-gloss surface repair spray.