Juno,
As Rick points out, car duco paint is very durable. It is designed to withstand permanent outdoor exposure in sun, rain, hot and cold for years. The spray cans are very expensive as Rick says.
1. You should rough up the surface with a nylon scourer to rough it up then wet sand the metal surface with 320 grade wet & dry paper.
2. There are numerous undercoat products, metal etching primers and which you only need if this is the first time the surface has been painted or if you sand back to bare metal. If the tube is already painted and you are repainting then you only need to wet sand, wash let it dry then paint.
3. To make a shiny finish, you'll need 3 coats of the paint then topcoat with 3 coats of clear finish.
4. if you are spraying, you need to strip the tube completely, Nuts, bolts, mirror and cell, spider, focusser and finders and their mountings.
This guy is a bit annoying to watch with his gun finger but he seems to know his stuff. One video has lots of handy tips, the other shows how to spray paint and lacquer to a shiny finish:
https://www.youtube.com/watch?v=7he7VBJaJ_E
https://www.youtube.com/watch?v=rn_J8CQDbGI
Good luck
Joe