Howdy Chuck!
I have an idea for masking the spinners - don't give up on masking tape discs, just cut them larger than the intended white area, them make one radial cut and shape them like a cone over the spinner. You know, so that the mask and the surface to be painted are parallel at the edge. That should take care of the overspray. If you like, I could help you calculate the exact cone, but it's easy, so you can probably tackle it yourself.
One more thing - you might want to pigment the tailwheel's whitewall a bit the brown side, I've seen them age like this on german equipment. This also depends if there's somethin iron nearby - like the wheel or the reinforcement wire in the tire itself. Then the whitewall gets a little rusty really soon.
I know you won't give up, looking forward to see more, greet the girls from me and have a nice day
Pawel