Have you tried using a resist first, then apply the paint that away if you do get outside the area you can wipe it off with a Q-tip, I use small foam cleaning pads they are about the size of a Q-tip but don’t have the fuzz, they are flat on the bottom. When you load your brush with paint wash start in the center of your work and let the paint flow towards the edges then work the paint into the edges with the brush. I keep a second brush handy that is damp if I get to much paint in an area I use it to soak it up, just rince it out and dab it on a paper towel.
Hope this helps
Doug