I read about condensation in automotive applications. The circuits in my instrument cluster are covered in a clear product, but what about the in-circuit fuses and those in the fusebox? Those aren't covered and yet they don't pop a gasket when I have to scrape the INSIDE of my windshield. Are 5V logic circuits that much more sensitive as opposed to 12V automotive circuits?
Think about condensation. Warm moist air on one side of a barrier, something causing a cooling effect on the other. It matters where the device is placed. Take the radio for example. Placed in the dash. The air around the radio for the most part will be the same temperature as the air inside the radio. The case having "holes" in it to allow for air flow. If the radio was placed against the window or an uninsulated metal panel then you would have a problem.
Side question: can I use ordinary water-based lacquer spray to protect the assembled circuit? Or do we really have to use the specialized products?
As long as it is dry before you power it up it should be fine. Anything non corrosive that will seal the board will work.