You can use several things to cut the masks themselves. I've got a buncha punch & die tools which I've used to make roundels in combination with post-it tape and also what's known as transfer tape.
If you haven't got any punch and die tools, get some varying sizes of brass tubing and use an X-Acto to trim away either the inner or outer edge of each tube (finishing off with sandpaper or files to even it out) until you get a nice sharp cutting tool.
Anyhoo, I'll get all my measurements worked out in advance (however you choose to do this is up to you) and cut several sets of masks as backups.
This is not to scale...
Gray represents a mask or masks.
1 I'll use the female outer mask, positioning it correctly, and then spray red.
2 I'll then place the male outer mask with a section punched out leaving the area to be sprayed white uncovered; I'll also place a mask covering the inner red circle.
3 After spraying the white, I'll remove the inner masks, leaving the female outer mask; this is the result.
4 I'll then place a mask over the white and red and spray blue.
5 Removing all the masks, this is the result.
For roundels with the yellow surround, or for the two-color roundels, adjust accordingly. The only other roundels I've done using this technique are Aussie, but you can use it for virtually any roundel you can think of.
Some may find this difficult, but it's EZ for me; and fairly quick. Any questions, give a hollar.
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