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.
Priming and Painting Galvanized Metal, condensed from KILZ.com, other manufacturers also make specific paints and primers for galvanized metal.
The galvanizing process, which is designed to prevent rust, leaves an oily film that can prevent coating adhesion. The zinc in galvanized metal can produce a milky “white rust” (which is common when it has weathered) that must be removed with a stiff brush or abrasive pad, prior to coating. It’s necessary to remove the oily film that can prevent coating adhesion with a water-based cleaner/degreaser.
Prime the surface with a water-based primer. Since oil-based primers can interact with the zinc in galvanized metal, causing premature peeling, always use a water-based primer when painting galvanized metal. KILZ® 2 Primer is an excellent choice when priming and painting exterior galvanized metal. It will grip to the metal surface and provide a better surface for the paint to adhere.
todayshomeowner.com/how-to-paint-a-galvanized-metal-door
Best Answer
I would avoid drilling into the door for a couple of reasons. It may void any warranty on the door and if the dog was able to get a corner or edge loose they may damage the door pulling at the plastic. Cut a piece of lexan or plexiglass large enough to cover the area of potential damage. It needs to be large enough so the dog cannot grab the top while on its' hind legs. If it is a large dog you may have to cut a hole to go around the door knob. If the door has a paneled design stamped into it drill several small holes in the bottom side of the Lexan. This will prevent condensation from forming between it and the door. I would then adhere it to the door with clear exterior chaulking applied around the entire perimeter of the panel. It may help to lightly sand the Lexan where the adhesive will be to get a better bond. Position it in place with clamps or duct tape until the chaulking has set.