I use a Testors brand brush made for model cars, found it at Walley World years ago. I use it to dye my sheaths, and yes I thin the dye for that, not sure about fading edges, never tried that. This brush uses CO-2 for the air supply, not the cheapest if you do alot of spraying, those little cans don't last long. I did see Harbor Frieght has the complete air brush with small compressor for around $79.00, I think. It looked like a good way to go.
Dale