I use 210.* It comes as a cleaner and scratch remover.
This is the site of the folks that make it.* As you can see from the site it is an Avaition product.
If you are at the point that it is cracking you may be to far down the road to salvage any of it.* But here is what I would do with the one that cracked already.* Take a small drill bit and drill a hole at the end of the crack.* Make sure you cover all of the crack with the hole, so judge the drill bit size as such.* This should stop the crack from continuing.