What I found works best for me is Rust-oleum Painter's Touch 2X Ultra Cover Paint + Primer spray. I paint both the inside of the blank and the brass tube, it sticks very well to both, and it covers in a single coat. I use black on most blanks, but will switch to white on light colored blanks that I don't want to tale on a darker hue.
Tube Painting - I slip the tubes over a long stick that's about 3/16" square. I rest the ends of the stick over my trash can then lightly spray until covered, rotate the stick 90 degrees then spray again...repeat until the entire tube is covered. I make pens in bunches and can paint 18" of tubes in about 20 seconds. Best to let them dry overnight, but I've shorten it to a few hours in a pinch.
Blank Painting - Blow out any grit from the hole in the blank. Simply hold the paint can nozzle about an inch from the hole and give it a quick spray, then do the same from the other end of the blank.
The advantage of this process over using Testors which I used at one point is it's faster, adheres better, covers better, and leaves a consistent thinner coat. Oh yeah...don't forget to ware disposable gloves when painting the blanks...it's hard to get off your hands.
Good luck with whatever you decide to do.