It is entirely possible to paint roundels with a brush and without any masking. And you don't have to be Rembrandt either. All it takes is the right step-by-step technique. The "trick" is to always be painting the inside of all the curves with the natural sweep of your wrist.
1. Draw the outside circle.
2. Draw the next smaller circle.
3. Using a thin brush (with heavy paint) sweep around the inside of the larger circle.
4. Fill in the rest of the ring with a wider brush going OVER the inner circle line.
5. After that ring has dried, redraw the line you painted over and the next smaller circle.
6. Repeat steps 3-5 until you have completed the roundel.
It really works very well. By the way, I've found that enamel paints work very well for painting markings (but of course they are not fuel proof).
Last edited by abufletcher; 02-08-2016 at 08:01 PM.