Well,
I had a nightmare once with getting any paint to adhere to a bunch of steel PSU chassis I made. Turned out that the steel I used had been galvanized (i.e. zinc coated) and the paint just ran off, or formed pools. Did a bit of research, and found that industrial equipment manufacturers use a special galvanized steel undercoat primer paint. Managed to find some in spray cans, and all was fine after that.
I'm sure there are other metal coatings/treatments that don't respond well to painting without being pre-treated with a primer.
Some chassis I've used have had a silicone-feel to them as if they have some kind of rust-proofing treatment? Watch out for a light covering of oil too.
This galvanized primer worked a treat for lots of difficult-to-paint surfaces though.
Hope this helps!
Mark