My advice is to simply use alcohol to clean before spraying. A harsh degreaser may affect the seals on the scope. And while some do lightly sand before spraying, I do not. Krylon will adhere to the scopes surface fine without sanding. Later if you decide to remove the paint to sell the scope or just to go back to its original finish, it will be scuffed up from sanding. I've had several scopes painted with krylon (Leupold, S&B, Trijicon, Nikon, and NF). None were sanded and the paint held up fine. It was also easier to remove when it came time to sell them.
Edit to add, do one light dusting coat first. Let it dry for a few minutes, then do a light, solid coat. Let it dry for a few minutes then give it a good solid coat. You can use a hair dryer between coats to help it dry. Just be careful not to coat too thick so you don't get runs in the paint.