I'm not sure it's absolutely necessary, but it should make painting easier if you make a little bit of a rough surface to help the paint adhere. But light hand-sanding should do the trick. I think a power sander could take it too far.
That's right, it's safe to glance over it with a sanding sponge. A rule of thumb is do not paint over a shiny surface without dulling it.
If the door is metal and insulated. Be sure that the door does not face the sun during the hot period of the day. It will warp if painted a dark color. Also dark doors that have storm doors placed in front will also warp if made out of metal. This will void any warranties on the door if any are still left.
As far as sanding,, be sure to wipe the door really well with tack cloth and use paint and primer for metal. Most likely if the door was never painted the white door was simply a prime color finish. As your supposed to paint the door once you purchase it. Most folks do not do this as the primer always looks pretty good when new.
I agree w/ Peace rub over it w/ a sanding pad.Something else to keep in mind.if this is in the heat try to get it done in the coolest part of the day.This helps with the edges drying to fast,before you finish.Make sure the sun is not on your door or the paint will almost dry right away.Good Luck
I spray painted a metal door black. Here's what I learned. Wipe door down with vin ear first. Use metal primer, let dry over 24 hours. Wipe with tack cloth and paint. Good luck. I do not want to tell you how many times I had to redo door until I got these tips from my father in law.