I did this a couple of years ago. A Stanley steel exterior door that had
been in place for 10+ years. I stripped it in place and got it nice and
clean down to bare metal, removing even the factory primer. Exterior side
only. Then I took it down (put some wedges under it to carry the weight
and pulled the pins) and took it into the garage where I kept it standing
upright. I wiped it down with paint thinner and got it very clean. As I
recall I didn't prime it but used several coats of good quality satin
finish canned spray paint (off white). It came out very nice.

For one day there was no front door and since there is no storm door I had
a plywood sheet cut to size and stood it in the opening, locking it into
place with a few sliding pin latches. I made a few little holes to accept
them.

Replacing the door was very little trouble. I thought it would be worse.
I just stood the door on some supports--whatever was handy--which I played
around with a bit until the height was exactly right and I could slide the
door into position with hinge parts aligning and I pushed the pins back
down; gave them each a tap of a hammer and it was done.