I'll be reading this thread to keep up with it. I painted my car in the garage in the past and found it to be tight and messy.
A buddy and I painted a 70 Ford Mach 1 under a wide carport about 2 years ago and worked out real nice. Before painting the first thing we did after wipeing down the car was to hose down the cement to keep the dust down and to keep the paint from sticking to the drive. I don't know if this will help you just letting you know how we did it.
Kurt