Galvanised water cans are sturdy and long-lasting compared to plastic cans, but they can still develop small leaks over time. A few small holes in your galvanised watering can don't mean you have to throw it away, though. Depending on the location of the leak, you can patch it successfully with a bolt and some washers or a specialised paint used for roofing and sealing metal gutters, and the repair will last for many years.
Scrub the inside of the galvanised water can with water, soap and a wire brush to remove rust and dirt. Allow the can to dry thoroughly.
Stick electrical tape over the hole on the outside of the water can. Pour a 1/2-inch-deep layer of asphalt paint into the bottom of the can.
Place the can on a level surface covered with newspapers in case of leaks. Let the asphalt paint dry and cure for six hours, or according to the manufacturer's directions. Remove the tape covering the leak.
Store the can out of direct sunlight. If the black asphalt paint inside gets too hot, it may become sticky.
Drill a slightly larger hole through the leaky area using an electric drill. The drill bit will smooth any sharp edges on the original hole and improve the long-term durability of the patch.
Thread a bolt with a slightly smaller diameter than the hole through a metal washer and a rubber washer. Push the end of the bolt through the hole in the water can.
Thread another rubber washer and another metal washer over the end of the bolt, sandwiching the metal side of the watering can between the two rubber washers.
Screw a nut over the end of the bolt and tighten it down with a pair of socket or box wrenches until the rubber washers are compressed. The compressed rubber on either side will plug the hole and prevent water trickling out of the leaky area.
Asphalt paint is available from home improvement stores as a roof repair product or driveway sealant.