More coats of primer might have done the trick. Paints have different levels of coverage/opacity.
If you wanted to even out the texture, that is harder; maybe some sort of skim coat over the entire wall, to unify it? Or pre-paint/joint compound/etc. over just the areas that were chipped.
I don't know whether you can add additional layers over your existing paint; the can may give more info about overcoats.
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.
Best Answer
Paints, in order of shininess from least to most, go: Flat-Matte-Eggshell-Satin-Semi Gloss-Gloss-High Gloss
10+ years ago: The reason shinier paints are shinier is because they have a greater concentration of binding agents (adhesives). Because greater amounts of adhesives provides a sturdier paint, glossier paints are more durable and washable than flatter paints.
Satin is generally considered a good middle-ground. It's flat enough to not show tiny imperfections but strong enough to endure washing.
Today: As the chemicals in paints have evolved, flatter paints have become more durable and washable (provided you're dealing with a quality paint). I'd wager the adhesives in matte paint today do a better job of binding the material than the adhesives you could find in a gloss paint from the 1990s.
Today, it's just a question of: How shiny do you want your paint to be, keeping in mind that the shiner it is the more imperfections it will show?
(Personal note: I'm pretty thrilled by this development. I never liked shiny paint and just a few weeks ago painted my kitchen with Sherwin-Williams Duration matte turquoise. Duration is one of their lines that offers washable mattes, and S-W has a very good track record to their name.)