Really the key to smooth as glass paint is all in the prepwork, and how you spray it.
Go through the sanding process like I described, and your paint will be ultra smooth.
You can NEVER get rid of every scratch on the surface. But, every time you sand it completely with a smoother grit, the scratches are that much smaller. When all the scratches are as small as you can get them, and of the same depth all around, your model will shine like no other. I've had color coats come out smoother and shinier than some I've seen with clear on them.
All in the prep, and control of your hand.
Corey