I don't generally rough with sandpaper, but then, I've not had any flaking or peeling from the paint jobs I have done. The one spot I see on the Cannon where paint has left is a place where it commonly rubs on the holster, so I'd expect wear to be the culprit.

I don't know what methods were used on your Vulcan, but the biggest thing I see with flaking paint jobs is that the paint was applied too thick, usually in a single coat (by hand or spray). The Thundergun above, for example, has something like five very thin layers. Two of a matte black as primer, two of each metallic, and then the clear coat.