It can be as simple as Super glue and a rag it helps to have real fast reflexes.

It needs to be applied with needle point accuracy and rubbed hard with a rag as quickly as possible.

If the crack is over hair line..The best way is to 'sand it in' with 4 or 6 hundred grit paper...you can apply super glue to the paper or the crack...just remember it all happens quickly.

Not the super glue at the hardware store buy the thin version at a hobby shop.