I have a metal garage door which has been painted many years ago. The paint was flaking with a similar effect to a crackle glaze. I have managed to scrape all the loose paint off, revealing a further layer of paint which is firmly adhered. Around the edges of the door are some small areas of bare metal and the original colour of the door. I will sand the door before painting but please could you advise me on the best type of paint to use and whether I need to use a primer and or undercoat . If so, what are the most effective & should these be used over the whole door or just certain areas. Thank you