Never done a door but as they are usually colorbond it is probably like re painting a roof. My roof was redone done in Weathershield (or perhaps it was called Roofshield, anyhow it was a Dulux product) with Emulsabond added (by the contractor). He claimed that EB would make the paint stick to anything.
So my suggestion is... prepare, prepare, prepare. i.e clean and sand the old surface well to get a good key, then pick the appropriate somewhat flexible paint such as Solarguard, Weathershield or whatever and probably add some EB. You might also consider priming with Zinsser (look up their web site) because that stuff really does stick to anything.
If you are doing the same colour, I would try doing it in situ as a removed roller door is like wrestling a gorilla and they are buggers to roll up again when re-installing
Anyhow, I'm no painting expert and others might have different views but that's the approach I would take. I will look forward to seeing other enlightened opinions.
Good luck![]()