interior case painting need help!

I have a chieftec dragon case i bought some paint to do the inside, and i painted a removable drive cage ( it think thats what its called ) and let it dry. then today i checked it out and it was very easy to peel the paint off.

I was told to use vineger to take off a protective oil and i did that and havent tried painting it again, but was wondering if i should sand it and use primer.
if i need to use primer i have another question. i saw on somthing where they primed some metal for a paint job then sanded it off. then i think they repeated that. anyone know why? should i do that?

thanks for the help.

i think painting and then sanding and priming over and over is used to get a really smooth surface. sanding and priming once, though, might help to get the paint to stick better.

without sanding.. just sprayed..
(its a low quality save.. but the paint is on there.. not glossy but smooth

what kind of paint is that? did it get scratched off by screws (like the drive screws)?

its model paint for a buch of surfaces, i noticed it had a scratch in it i dont know how, so i scratched it with my fingernails lightly to see if more would come off and it peeled right off