I have a wonderful old door and it contains three small glass windows which I generally like: light shines through, I can see outside and they just look nice optically (from the inside and the outside):
My only concern is privacy: it’s easy to look inside from the outside. Is there any privacy shield (maybe paint or something that I can cut) that preserves the look, allows looking outside, sun to come inside but not to look inside from the outside?
You seem to want a one-way mirror, but this only works if it's bright outside and dark inside. At night, when it's dark outside and your house is bright with lights, people will be able to see.
If you want a solution that works at all times, buy a bottle of "Frosted Glass Spray Paint", mask the door with tape and paper, and paint the glass on the inside with the frosted paint. I find that it lets enough light in, and looks good from the outside -- although it does not let you look from the inside to the outside.
If you only need privacy during the night, then you could just install some outdoor lighting, and point a spot light to the door. That will do the trick.
