My sister had he front door professionally painted in multi surface, satin black about a year ago. Despite my scepticism it looks really good and most people just think it's a new black composite door. How long it will last I don't know but still looks as fresh as the day it was done. I think longevity will depend on how well the surface was prepared first.
Sorry just noticed this. I don't know exactly just for the door as he done a few other bits while he was there. He charged £200 for the day for him and his assistant for labour, a bit on the high side I thought but all his work was rubbed down well and had a good finish so I thought better than two idiots for £150. The paint was expensive around £40 for 2.5 litres ( AkzoNobel multi surface) but she has tonnes left and is saving it to do some other jobs around the house.