I also have a small paint dropper bottle with Windex glass cleaner (just the normal blue stuff) which another painter showed me years ago. It works very well as a as a medium that acts as a slight drying retarder, though also very well for a surface tension breaker for washes and paint thinner for regular blending and highlighting, so I now use this almost exclusively instead of water or saliva.

So can you or would you use this to thin paints back down when they have dried out a little?? I normally just add a few drops of water, but if windex works and add's value at a drying retardant then hey, that's cool.