I use a little etch primer, out of a spray can is fine. The primer will help adhesion but mostly it helps your top coat cover.
Some gray primer will allow your mopar blue to cover quicker and more evenly.
The factory didn't use primer because they didn't care. Same reason they had over spray on the manifolds.
The engine only had to look good enough to get the car sold. What it looked like 3 month later didn't matter.
