I am still to take time with my air brush but it is still very much on my agenda. I simply placed the backer board on some paper and spray the main color on first, not caring that the very outside section is covered very well, (only lightly), then while it is still wet I take a can of spray black and spray it so that the center of the spray is actually spraying on the paper and the rest is going on the backer board. I place the BB so the the part I am spraying is farthest away from me and the turn the BB so the next side is farthest away from me and continue with this method until I have done all four sides. This results in a sort of blended pattern for the paint, as the most dense spray is hitting the most outside of the BB and the ' over spray' is in the center part. If I get the black closer to center of the project I just respray the center color so that it is the center of focus again. Give it a try. I certainly like the outcome of this method.
Dick
heppnerguy