Hi Dchipy, hard to reach spots are usually also not very visible. So I would use a black primer to start. Spray can primer usually gets in all the cracks. If not then you can go in with a brush. Do that before you start painting the surroundings so it doesn't matter if you hit some other parts. If you don't want to paint over black in general you can spray some white primer top-down on the model so the recesses stay dark and the rest gets lighter.