Matte paintings were used before the dawn of competent computer graphics to simulate a larger/more dramatic/exotic location than can be achieved in a film studio. Paint was directly applied to glass, which then sat between the camera and the actors (leaving a clear section to capture them).
I would imagine putting it on glass also made it possible to remove the paint easier if a mistake was made, wouldn't it?
Yes probably, you could scrape with a knife or spatula.