First did you wash and scuff sand the door?

OK since you have a mess, get some xylene at the hardware store. OUTSIDE..put down same cardboard and set the door on it. With a brush...brush the xylene over the door and use the solvent to wash the paint off. This will take a while.

Clean it off with a rag. When it dries, scuff sand the door with some scotch brite or #220 sandpaper. Prime the door with exterior acrylic primer. Make sure that you buy a GOOD brush not a $5.00 cheapie. Also by some latex paint conditioner to add to your top coat.

I'm not sure of the kind of "steel" doors you are talking about...but I just repainted a couple of steel security doors using acrylic enamel automotive paint. This of course after thorough cleaning, sanding, etc...prep work before laying down the primer and top coat.

Greetings, I'm with the great advice of the above two members, but for a high-use object (especially steel), I'd use alkyd-based materials ONLY. Granted it's more difficult to work with, but it goes on in much thinner coats and dries much harder than latex. Latex is good with drywall & wood because those materials often shift, but alkyds are much stronger and are ideal on material such as steel, which as in steel doors, that steel won't be shifting. And if it does, call your retailer! Dave