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.
If you select boards with knots, there is a good chance that they will bleed through, even if you use a sealer or shellac based primer. The resins in knots is persistent.
Clear boards are usually used for moldings, including baseboards. You could also use MDF (medium density fiberboard) or solid vinyl composite molding. When painted, they are hard to distinguish from wood in most applications. (I think they look a little too smooth and I prefer wood). For painted applications, there is also fingerjoint molding, which is made of interlocked short pieces and is cheaper than clear boards. Much pre-primed molding is fingerjoint. Occasionally these joints may show if the quality is not high.
For wood or MDF, a light sanding with 120 grit paper is recommended to remove milling glaze and to give the wood a tooth to better take paint. An orbital sander is fine
Then prime and paint. Many prefer oil based primer on raw wood and MDF. Some vinyl says it can be painted directly, but I would also give it a sanding (or avoid it unless necessary, such as in outdoor uses).
Latex paint is fine for a finish coat, but it tends to look less smooth than oil (alkyd) based finishes. Adding a leveling agent like Floetrol helps, but nothing lays as smooth as a good oil based enamel.
(P.S. To me, using a really smooth straight clear board, in furniture or in molding, is appreciating and honoring the wood, the tree, nature. But to each his/her own.)
Best Answer
To do this right take the door down.remove hardware, remove gaskets. lay door flat on saw horses, apply a exterior oil based primer to all raw wood with a natural bristle brush 2.5-3”.Allow to dry Rehang door and wait 24 hours.
Remove door again lay flat on saw horses. Sand the primed wood carefully smoothing the raised grain.
Dust off or blow off all surfaces. Paint with high quality exterior 100% acrylic paint of the desired color and sheen. Use a real nice nylon brush like a Purdy 3” $20.00-$25.00
Let dry. Rehang door. Wait 24-48 hours. Remove door. Repeat the sanding and blowing off dust. Apply second coat. Let dry. Rehang door reattach all hardware after 24-48 hours.
During each painting step be sure to coat the top and bottom of door.
After priming frame and jambs apply wood putty to any holes allow it to dry then sand. Apply caulk to any gaps after primer has dried. Allow caulk to dry before painting.
After final coat has dried for 72 hours reinstall gaskets.