Where did I go wrong with this frame painting procedure? I just painted a bike frame (steel) with the steps outline below. However, every glancing contact with a rough surface immediately strips paint from the frame and I can see the metal shining through. This is the second time I've made this experience. I had expected that DIY paint may not look as great as a factory finish, but I had at least hoped to have it be durable enough that I don't need to worry about scratches every time I lean the bike against something.


Removed one layer of old paint with paint stripper
Removed second layer of paint (not affected by stripper) with sandpaper. Down to bare metal in many places, but not everywhere.
Wet-sanding with 1000 grit for the entire frame to make it easier for the primer to hold firm
Clean with water, dry with towel and air, then use brake cleaner (supposed to leave no residue) and alcohol (the kind you can use for cleaning or making fires or whatnot; 'Spiritus' in German). I was hoping this would remove all grease from fingers and such
Apply 1-2 coats of primer
Let it dry for 24h
Wet-sand frame again with 1000 grit
Wash, dry, clean again, same as above
Apply base coat in 2-3 coats, letting it dry for at least 30 minutes between coats (paint is supposed to be non-sticky after 5-10 mins, dry after 20-30 mins, fully cured after 24h)
Apply several clear coats (same intervals as 9.)


The primer, paint, and clear coat are linked here (in german).

Now I realise this is about the cheapest spray paint you can get, but I had hoped that I did the procedure correctly. The frame has been resting for about 12 hours when I scratched it, but I don't think it'll get much harder, judging from my other frame I painted.

Hypotheses: 


The alcohol I used for cleaning wasn't good
I should let each coat of paint dry 24h before painting over it (doubtful)
Paint is shitty


Where else could I have gone wrong?
Here are some pictures of the finish. There are some spots where I wasn't careful enough and it looks like some white powder stains the section, but over all I'm pretty satisfied.





Here is one of the two spots where I touched the frame against a stone wall. It's not well visible here, but it looks very silvery as opposed to gray (the primer color), so I think it's down to the metal.





One shot from the bottom. The gouges are an artifact of insufficient sanding, not of the paint job I think.
Painting is tough and the artist in me (tongue-in-cheek) is never satisfied with finished product. Your prep seemed meticulous enough and you express satisfaction with the finish where it hasn't been marred. Given the work put into the prep, why go with bargain paint and risk the poor quality of it?  Give you props for using the same "brand and series" of paint.  That tip is frequently included in DIY paint tutorials. 
The two most popular brands of spray paint here in Midwest, USA are Krylon and Rust-Oleum. I bring this up to share a couple of things common to the directions for use printed on these cans. Despite the myriad of differing formulations and applications within a brand here are some common advisories they share: Priming is especially recommended on bare metal or wood.  Apply multiple, thin coats, waiting a few minutes between.  Apply additional coats within one hour, OR AFTER 24. Some types will state "recoat (or apply clear coat) within one hour OR AFTER 48."  This is under ideal drying conditions of 50% humidity and 70°F.  Finally, it's commonly noted that on plastic, "maximum paint adhesion and durability is achieved in 5-7 DAYS."  I realize your bike is not  likely to be plastic but I think that tip can lead us to believe that the paint will continue to cure and become most durable quite some time after the "ok to handle in such and such hours."  If your paint job is being marred right down to bare metal as you say, the paint is not nearly cured enough. Certainly 12 hours isn't enough time to expect it to hold up.