Wolfgang, I clean the bottle thoroughly, then use a rotary burr to remove material along the crack to about 2/3 the thickness of the material. Drill a .5 mm hole in the ends of the cracks. Fill the groove with your 2 part epoxy of choice.
Here's one prepped for filling. This one I did a few weeks ago. The one on my bike was done with the same method 4 years ago and still no leaks.