Your powders fade because you're blowing them back off the model with the rattle can (or the airbrush, to a lesser degree). I've stopped using straight chalks, and gone to Pan Pastels. The process is: 1: flat coat first 2: airbrush a basic weathering scheme. Dust/earth on the car body, soot on the roofs (I'm a steam/Alco guy), and weathered or grimy black on the floor. 3: accent with chalks or powders. I've gone mostly to pan Pastels, but still use some AIM and Doc Bragdons. Rembrandt chalk sticks are also very effective. Their pigments are denser. You don't lose as much when sealing. The cheapy pastel stick sets from Wal-Mart don't work very well in my experience. Yes you can make them work, but you'll get better results in less time with better quality materials. You get what you pay for with this stuff. I never skimp on the weathering materials or paints. There are better places to save money. I always airbrush the flat coat on. I don't like rattle cans for fine detail work They have two settings: off and full blast. You can throttle with an airbrush. I only use rattle cans when priming resin castings, loose lumber, or very large areas. Window glazing last, or mask, or use an acrylic flat coat and clean with a cue tip and thinner. RR equipment quite often had dirty windows, so I don't obsess here. Variety is the spice of life.
