As with most everything else, preparation is everything! Make sure your cases are clean. Give them a bath in your bathtub. Dry them well. From this point on, use rubber gloves to keep oils from your skin from getting on the cases. Once you are sure they are dry, spray them with lacquer thinner to get any residual oil off. Let the thinner dry a couple hours. Then you are clear to paint. I have had great results with this.