Below is what I do:
1) Sanding with 2000 grid sand paper. I use a piece of wood as sand block, and I finish by hand the part not accessible with the sand block.
2) I use an alcohol based cleaner to remove any sanding dust and oil/grease from the surface
3) I apply 3 thin layers of primer, about 10 min interval between each application (3 layers is what it takes so I don't see at all the bare metal)
4) I wait about 30 min after the third layer of primer
5) I apply 5 thin layers of paint , about 10 min intervals between each application
6) I let it dry at least 24 hours before to manipulate it by hand. But you need to count 48 hours to consider fully dry.

Note:
- I start directly with 2000 grid because I'm painting bare metal. If there was paint, I think you'd need to start with a lower grid (300-400) to remove the paint and then go progressively go higher in the grid to get a smooth surface
- If you go for a shiny mirror finish, I think you would need to sand slightly between the primer and the paint, and also after the paint to remove the slight "orange peel" texture due to the spray painting. You would also need to apply a clear coat at the end.