FrozenPea, now this idea takes more luck then it does skill, but it works for me most of the time, and if it does go a little further then I want then I cover with the base color.
After thinning the paint, and here comes the luck part, is getting just the right amount of paint on your brush, then hold it straight up, then dab your piece from underneath, then the paint spreads by itself.
Just recently I've been doing 2 hummers,and on their transmission pans the first one I had just a hair to much paint and it spread above the line a tad, then the second one, I put just a little less paint, and was just right, it stopped right on the line where it was supposed to.
When done it looked like somebody spent a lot of time to keep in-between the lines, but it only took me a second.
This is the same principal as dipping into the paint, but you control the amount of paint your dipping into so not to go above the line.
I hope it helps you as much as it has helped me in the past. Good Luck
Kerry