Re: painting an exterior metal door
The paint likely failed due to poor or lack of initial prep. I would recommend making sure all of this coating is removed before starting to paint.
Proper prep would be to wash the door down with Tri Sodium Phosphate (TSP ), which will clean any dirt, grease and grime from the surface, as well as slightly etch the surface for better adhesion of your new finish coats. Rinse well after washing and allow to fully dry. Apply a good primer, such as Zinnser Bullseye 123 (blue label ). Primer sticks to the substrate better and creates a better binding layer for the paint to adhere to. I would also tint the primer towards your finish coat color to help reduce the number of coats and the coverage of the top coat.
I would stick with either a latex or an alkyd paint. Latex is easier to work with, alkyd is a harder, more durable paint (replacement to oil based paint ). You can paint the door while on the hinge, but you will need to remove the weather strip from around the door first. This will allow the door to be closed while wet, yet not be damaged by anything touching it. It will be best to apply the paint in cooler temperatures and not in direct sunlight. You can use a roller to apply the paint, then a brush to lay it off (smooth out the roller marks ). Working quickly in wet paint will allow you to get the finish on the door with minimal streaking and brush marks. Give a light sanding between coats will help to reduce brush marks as well. Once the door has cured/dried for several days, you can reinstall the weatherstrip around the door.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!