I've tried painting tubes with acrylic paint and brush but I found it went on unevenly and too thickly which meant the tubes might not fit into the blank holes, or might scrape the paint off while being inserted. I settled on using car spray undercoat in white or black. It was so easy to get a nice even coat and, if I also sprayed the insides of the blank as well, I found it coated them nicely without being too thick a coat. Just make sure the brass tube is keyed suitably for better adhesion. It also has the added benefit of being quick drying, but I usually left it 24 hours to make sure. I also made myself a thing to hold the painted tubes or blanks, using a piece of board, ply or mdf, whatever is to hand, and using some rods in a grid, covering the board to make it like bed of bails. I could then drop the painted tubes over the rods to keep them held apart and protected while they dried. I used a load of nails for mine as I had them to hand.
