Ouch! ....... I like a two part epoxy, because it flows a bit, but generally won’t leak through to the front. Before applying to the rear, I take my Dremel and create a shallow trench down the crack (on the back!). I have a small, metal, cone shaped bit that is perfect for the job (check the selection at good hardware store). This creates a better surface for the glue to contact both sides. After that, I go through my grand daughter’s huge selection of coloured markers, and find the right one to touch up the flaked paint on the front of the crack........ good luck.