Ive been using the premixed joint compound that is in a plastic bag, and is put in a cardboard box. Every time i use it i make sure that i close the bag good. When i go back to using it the next day or so, some of it dries to the side of the bag, and crumbles in to the rest of it. What can i do to prevent this? Ive noticed that this stuff is pretty thick to begin with. Do i need to add a little water to it before using it?