I always have good results when I do the following:
1. Add a few drops of Kodak Photo-Flo to eliminate surface tension. Available in photo shops that sell photo developing supplies.
2. Add a few drops of Acrylic Air Brush Medium to retard drying time so that it does not glog the air brush tip. Available in most art supply stores.
3. Then thin the paint to the proper consistency with the manufacurer's thinner. Or, better yet, a 50/50 mix of distilled water and denatured alcohol.
I hope this is what you are looking for. Good luck.
Pete