My bike after a new spray paint, gets scratched easily I completely sandpapered my bike a few months back and repainted it with a coat of primer first, and then matt black. The paint was sprayed only after sandpapering the bike with the appopriate gritt so as to make sure that the primer would stick great.
With the original paint, my bike hardly got scraped too much and too badly, but now, it is easily scratchable. The pure metal is visible as if the primer and the matt black simply were peeled off (where the bike got scratched).
What could I have done so differently to make it so much more scratchable than the original factory paint job?
More Info

I sandpapered the bike clean and wiped it down with rubbing alcohol as suggested (I don't remember where).
Avoided touching the frame before the painting.
Sprayed a coat of primer.
Once dried, sprayed the matt black paint.
The paints were not at all sprayed heavily to make the paint peal off, and not too lightly either.
Commercial paint jobs often use heat-baked enamel, with multiple coats (not just undercoat and topcoat), and finished with a clear top coat.

The baking process produces a really tough, well bonded coating, and the clear topcoat (as well as being tough) produces a nice finish that hides scratches.