That's real materials problem. It sounds like the finish never adhered properly to the base. Call or write Glock and ask if they will redo it for you. They might do it free, but you'll still be out the cost of the shipping. You can also ask if there is a solvent that will attack that finish but not the base plastic. If not, I'd guess you might be down to some form of particle blasting. When paint has to be blasted off wood or other soft substrate the blasting medium is soft, like the granulated corncob used in case cleaning. You need the right blasting equipment for it, obviously.
