I am perhaps a little late to the party, but I would have to ask, how long did the paint cure for? And yes, I mean cure, not just dry.
In my experience, Tamiya flats are soft and easily scratchable hours after being "touch dry" which under some conditions can be as little as a few minutes.
If you gave it, say 12 hours, (preferably a bit more) I think you'd find the paint much harder to scratch.