I once used decals and scotch tape as a sort of reverse mask. I needed a perfect circle in two or three places on a wildly painted assembly ship made from a Monogram B-24D. The build required two white circles on the vertical tails, and one large black one on a wing. Then, a letter code would go in the circle. Lacking the tools to make a proper mask, I found I had British roundels in the perfect sizes, so I painted the tails white, the area on the wing gloss black, and applied the decals. The I painted over them with white for the wing (the aircraft was white with blue and red polka dots from the wing trailing edges to the glass nose, and OD with Orange and yellow dots from the trailing edges back). Anyway, I painted over the decals when I painted the overall airframe, and when I was done, I used tape to rip up the decals. I got perfect circles, no muss, no fuss. But I was using MM paint thinned with that awful caustic foul-smelling, but very handy, Dio-sol, so the paint gripped very well and was not likely to be pulled off with the decals. I wouldn't try it with acrylics. But the same tecknique can be used for other painting purposes as well.
TOM