I would suggest that you don't paint the shaft, but coat it with grease, since the grease will help prevent the rust, but even if it does rust, it will be easier to remove the rust when you have to service the bearings, than it will be to remove the paint. The shaft has tight clearances to the bearings, and paint will make it more difficult to remove the shaft through the bearings. You could spray the shaft after it is assembled with PDRP (Cosmoline), since it is easy to remove the Cosmoline with some gasoline, should you need to. The Cosmoline will prevent rust.
If you want to use a grease on the chute plastic ring, consider Dow Corning silicone grease, or you could even paint it with never seize. I have never put anything on mine, and it has been working fine without any lubrication for the past 10+ years.