Has it ever been primed and or painted? with what?

The door is the original paint from when we bought the house, which was new. I assume its the factory finish.

It most likely is oil based. Lightly sand, clean, prime with a good bonding primer( Zinsser's 123) paint away

The usual steel door is galvanized, primed, and painted with (usually) a polyurethane before forming. That way the door company can form the door with rollers from prefinished coils of steel and won't need any paint facilities at all.

You don't say why it needs paint. If it's peeling it'll probably continue to do so even with new paint. I spray old doors, after cleaning and scraping, with ordinary airless equipment and good house paint. Nothing fancier than that. That's a lot of area to paint, and a flat paint shows fewer dents, but gloss and semi-gloss make most people happy.

Dents are a crapshoot, but small sharp ones fall easily to auto body tools like hammers and dollies, but they usually take two people if the bad spots are not on the bottom or top panels. Patching is with Bondo type fillers and a really good sander if you don't want to be doing it all day.

Don't close the door until you're way sure the paint isn't going to stick to the jambs or trim, especially that trim with the vinyl flap in it. Also remember that paint's heavy enough to mess with door balance, so disconnect the opener and run the door later by hand to check.

The door isn't peeling, its just that the paint looks like its "worn away" if that makes sense. I have to admit, we've never painted it, and we've lived here for 25 yrs. Its otherwise in good shape, no dents that Ive noticed. My husband has done some auto body filling, so if on closer inspection we find small dents, I think he can fix them. Thanks for the tips, now we just have to wait until we get a few days of dry weather.