very small coolant leak?

Hello All,
My Gcar regularly needs topping up of the coolant. I can see where coolant is leaking from. There must be a very very small crack. I tried super glue, but because so much heat is released from the surface, the super glue cast just comes off after a while.

Does anyone know what I can use to seal the very small hole, maybe some kind of special resin or something?