I spray the backers in an open garage. Made "about" 8 wood pyramids with the bandsaw, about 1 1/2" high. Set a large cardboard on the floor and the propped up piece on that. Usually spray down at about 45 degrees. You can go immediately to the opposite side and do the same. Because the piece is flat, nothing on the top will run. Wait 10 minutes and repeat on different sides. If it's a complex piece, I end up spraying from 6 directions. Spray paint shot at any distance turns to a dark dust that can be swept up, so getting paint on the floor isn't a problem, although my cardboard is large.
If you try to spray outside, WIND IS YOUR ENEMY !!
jerry