For very small items, I just use a very small brush, but a very good very small brush. Over the years I have tried many different kinds (including a so called 20/0) and the one I have picked is a Kolinsky Sable 000. They are expensive litle devils, but they really do work better than any other brush I've used. The ones I have are made by Winsor and Newton and they are available at art supply stores. They keep a sharp point, spring back really well and hold lots of paint.
I do not prime small parts either, but I suggest you do clean them with alcohol or some such substance to make sure they are perfectly clean just before painting. One more thing - on occasion to make a really tiny dot, I use the tip of a round toothpick.
Rich