Sometimes, us modelers will have to paint circles on our models. Sometimes just for looks, somtimes to cover up markings such as Roundels. I found a very neat and easy way to do it.
1) Find a rubberband the right size of the circle you want.
2) Place the rubberband on the model where you want it.
3) Carefully brush a liquid masking agent(MicroMask, Ambroid Mask) around the rubberband and position the rubberband if needed.
4) Allow the masking agent to dry and brush on a second coat.
5) Paint the circles(brush or airbrush) and remove the rubberband and masking agent.