Doors – Remove plastic film from the outside of a metal door

Do I need to sand or is a good wash down enough? Or maybe a liquid sander?

A good wash, followed by a thorough drying, followed by a light sanding with fine grit sand paper to rough up the existing surface. Sanding ensures the new paint will get a good grip on the old paint.

Do I need to prime, or is the old paint (cleaned a bit) good enough?

No, its been sealed long ago - just clean and sand.

Is there any special paint, other than exterior trim (probably satin or semigloss) to consider?

Since there's already latex on it, exterior latex should be fine.

Is there any additive I should use with the paint for flow, finish, hardness?

No.

Do I really need more than 2 coats?

If it were white over black- you might need 3. Black over white, you might only need one good thick coat.

Is there something else I am failing to ask/think about?

Get saw-horses. Lay door flat on saw horses. Remove door from hinges, remove all hardware, tape all glass. Paint ONE SIDE only at a time, let try completely, then paint other side.

its kind or hard to tell from the photo, but this looks like haircell finish, not matte. its best identified by little swirly bumps, and is a really common finish on many plastic products (ironically for its ability to hide scratches).

if its haircell, there is not real way to remove it short of hours and hours of mechanical burnishing with burnishing rifflers, and they will cost you a fortune if you don't own them already. you also have to be able to actually do the work, and its an art form.

if its matte finish plastic, the best compromise will be sand the area smooth, and then to about an .25 micron finish (approx 400 grit wetsand). then tape off the whole area and beadblast with glass bead. you will have to experiment with different blasting media to get the finish you want.

either way, its probably cheaper to just replace the piece altogether, or have a new one fabricated.