Usually produced as custom material. Continuous manufacture, produced by a technology much like that used to produce plate glass, is of even thickness and has fewer flaws. Acrylic paintings on both paper and canvas should be glazed. Acrylic paint is sponge like and cannot be cleaned. Condensation can occur, usually due to humidity entering a framed piece through the back, especially if the piece is hung on an outside wall.
Cool air holds less moisture than hot air.
Glass has sodium in it which can leach out and attract water. Surface effects created when two smooth things are placed upon each other. Newton rings, those disturbing halos that are frequently visible on photographs when the glass gets pressed tight to it, can beget ferrotyping, where the surface of the image actually marries to the glass. Like attracts like in this way as well. Acrylic paint and acrylic glazing employ the same solvents and are very susceptible to sticking together unless properly spaced.