I have painted windows successfully without adding detergent. I have also
used powdered detergent, just to add "body" to the paint.
A friend of mine with a problem similar to yours solved it by covering the
window with paper. In the center he cut a hole at eye level, just large
enough for a face. Inside, he painted a monster with horns so that the room
occupants got a laugh at anyone who chose to peek in.