I find those sharp double ended toothpicks put down a smaller amount of paint than any paintbrush I can find. They don't hold much paint, so it is slow going to paint a very long line, but it works.
To control the amount of paint I put on the end, I shake the bottle of paint, then remove the cap and put the cap down upside down on my bench. A thin layer of paint remains in the lid. I dip the end of the toothpick in that layer, then touch it to the part I am painting.