The primary purpose of primer is to prepare the surface for the top coat. If the part has been blasted and has no dents or things that need filling, just smooth the primer down with a green Scotch pad which I believe can be found in grocery stores and home improvement stores in the paint section.
Don't use sandpaper because you are very likely to sand through the primer on the corners and edges. A Scotch pad is cushioned and you just want to knock off any lumps or dust that got on the part.
Regardless of what you are painting, the finish is no better than the prep work. If you keep that one fundamental fact in mind no matter what you paint, it will be good.
Surface prep is all about details. Be meticulous with those tiny corners and crevices especially on detail items like your vent frames. What you want is a part that brings you pride every time you look at it and that vent frame is going to be just a few inches from your face every time you're in the truck so do it right.
