How to seal the drain hole in my firebowl?

[Bubbleicious1]

I want to turn my black cast iron firebowl into a birdbath/reflective pool. The drainage hole is about the diameter of a cork. Whatever I use it must be black because the bowl is very shallow and I don't want a white/coloured dot visible in the centre. I obviously need a good seal, any ideas?

black silicone rubber bathroom sealant?

Put clingfilm over the hole from the inside of the bowl, and support it with a sponge, so you can press the sealant in from the outside and smooth it before it sets.

I expect the iron will go rusty.

If you want, you could wire-brush it and treat and paint it (not hammerite, please)