If the tube is already painted and you are repainting, use wet and dry sandpaper and sand it down with water. If it is a first paint onto raw metal, using a metal etching primer. You can put a layer of rust stop over that and then the final colour coat, The final paint layer can be an exterior house paint that you apply with a brush or a roller or you can shop for car duco spray paint at an auto shop which can give you a beautiful mirror gloss finish. Done properly, these will last for many years because they don't get the daily harsh sun, rain and nightly cold temps that they are designed for. I painted mine this way in 1980. Hasn't needed a re-coat in 37 years.
Have used Appliance White spray enamel and a few other colours of spray enamel quite successfully on my scope tubes. Just let it cure for a few days after coating before you start handling the tube, gives time for the paint to develop a harder skin.
Will still chip if bumped against sharp or hard objects so gentle handling is advised ( goes without saying ).