Birdbaths have a tendency to crack after going through a freeze and thaw cycle. The cracks occur because of the porous nature of concrete. Water is absorbed deep into the concrete, and when it thaws after a freeze, you end up with cracks. Small hairline cracks can be sealed with a waterproofing concrete sealer or epoxy, and larger cracks can be filled with a polyester resin. All of these products can be purchased at your home remodeling center, and will not create a health issue for your birds.
Fill your bucket with warm water, and add the appropriate amount of concrete cleaner. Your bottle of cleaner will inform you how much cleaner to add to your water. Some concrete cleaners do not require that they be mixed with water.
Empty your birdbath of any water that has not already leaked out. Extra water will dilute your cleaner.
Pour half of your cleaner into your birdbath and use your scrub brush to clean the entire bowl area. Dump the cleaner out, and use the remainder of your cleaner to scrub your birdbath again. Rinse your birdbath thoroughly with your garden hose. Allow your birdbath to dry for at least two days after cleaning it. Any water deep inside of the concrete will rise to the surface and dry out. This step will eliminate failure of your repairs.
Use a concrete sealing paint or an epoxy paint to cover the entire bowl. Brush on a light coat of the product of your choice with a paint brush, and allow it to dry. You do not want either one of these products to puddle. A puddled area will create a weak area in your sealer. Apply a second coat and let your birdbath set for at least 24 hours, or however long your product instructions require, before introducing your birdbath to water again. The drying time helps increase the bond of your sealant.
Repair large cracks by scooping out a golf ball size amount of polyester resin onto a stiff piece of cardboard. Squeeze a dime size amount of cream hardener on top of the resin, and use your plastic spreader to thoroughly mix the two products together.
Take your plastic spreader and scoop up some of your resin mixture. Spread the resin over the crack or cracks. If the resin sinks into the crack, apply a second coat. Once you mix the hardening agent into the resin, you will have a very short time to spread it.
Allow your repairs to dry, and then use your sandpaper to sand the surface of your repairs. At this point you can leave your birdbath as is, or you can go ahead and coat the entire bowl with a sealant. If you coat your birdbath with sealant, you are less likely to have more cracks appear in the future.