For really small detail, I use a toothpick- those double ended ones that come to a small point. I shake my paint jar, take the top off and set the top down upside down on the bench. Now, I dip a toothpick tip into the paint remaining in the cap. That puts on a little bit- just the right amount- of paint on the tip.
Not much paint is on the tip- you have to refresh it frequently. But that is a plus for working with fine detail. It will not flow out when it wets the surface and make a big blob.
That end of the toothpick only lasts one session. Once any paint hardens on it, it doesn't work as well. You can then use the other tip next time, then discard. Toothpicks are so cheap, and have so many uses in modeling, that I always keep a supply in a dispenser on my bench.