I want to use it as a bird bath.
This might help you: https://www.dukemanorfarm.com/2012/07/birdbath-makeover-tutorial.html
An epoxy would work!
Unexpected- Epoxy MAY not work well as it is sensitive to heat (sun light). It tends to turn to powder from high heat. I learned this while racing 1/12th scale battery powered race cars. the heat around the motor and batteries would cause the Epoxy to fail and turn into powder.