You do understand that painting your discs where the wheel fits them is a really bad idea unless you somehow got a uniform coating(electrostat powder coat?) as the thickness will be different all over, the wheel won't run perfectly true with the hub. You should really wire brush the surface on the wheel that butts up against the disc too to get rid of corrosion...
You can buy brake discs with a coating to stop them rusting where the pads don't run these days, would look and work considerably better than painting them...
The paint is soft and once the wheel nuts are all torqued up the wheel will sit level. You're talking about such fine tolerances that the wheel casting will be more out than any affect that painting the disc face will create, not including the general movement of the tyre heating/ cooling as they never ever sit dead centre. Hence for the need to balance them, and I think most weights go down to 5g?
I've done the same to one of mine to see what it looked like and it's been fine. Yeah the paint looks thick in the pics, but once it has dried it will be fine. If you're really that worried about it you could always paint upto the edge where the wheel meets the discs hub