I bought a vanity sink over two years ago and just installed it. Well when the mold was poured there must have been a air bubble when they poured it, in the bottom of the sink where the sink overflow runs into the drain ( about 1 1/2 inches from drain). It is about half the size of an pencil eraser that I need to patch with something.

My suggestion is fiberglass. Kit at auto parts store - follow directions.

I bought a vanity sink over two years ago and just installed it. Well when the mold was poured there must have been a air bubble when they poured it, in the bottom of the sink where the sink overflow runs into the drain ( about 1 1/2 inches from drain). It is about half the size of an pencil eraser that i need to patch with something.
You gave us all the information except the material the lavatory is made of. Plastic, fiberglass or china? Regards, Tom