Resealing a leaking 5-gallon metal pail involves the use of some type of sealant, typically the same sort of sealant that's used to repair a leaking metal pipe or roof. This can either be a water pipe tape or epoxy.
To seal the leak using water pipe tape, prepare the leaking area by sanding the surface around the leak with a course file rasp. This gives the tape some teeth to grab on to. Next, cut a piece of water pipe tape to the size of the hole, immerse the piece in another pail filled with water, and then squeeze it thrice over a 20-second period to activate the tape's adhesive compound. Place the tape over the hole, press it evenly until the adhesive holds securely to the metal, and leave it to dry.
To plug the hole with water pipe epoxy, break off a piece of plumber's epoxy that's the size of the hole you want covered. Massage the epoxy using your hands until the color is uniform and the material becomes pliable. Place the epoxy over the leak, and gently press it down until it becomes flat and covers the entire hole. Leave it to dry.
Though a water pipe tape or epoxy can be useful to cover a leak, these are only temporary fixes. A permanent solution means replacing the leaking pail with a new one.