Wood filler is basically a mixture of glue and gypsum or sawdust. The glue creates a binder that holds the material together for filling in small holes and cracks in wood. The filler is kept moist either with water or solvents. One way to tell the difference is to open the can and smell. Water-based fillers do not have a strong chemical odor. Often filler ends up staying in the can longer than expected and starts to dry out. Softening wood filler in a can is a matter of lubricating it with its moistener.
Open the can and pour just enough acetone to cover the top of the wood filler. Poke a 1/4-inch dowel into several spots in the wood filler. This creates channels for the acetone.
Add some more acetone just until the top of the filler is covered. Close the lid tightly and allow the acetone to work for 15 minutes.
Reopen the can and mix the wood filler around with the 1/4-inch dowel rod. The filler is difficult to mix at first but begins to move freely once the acetone mixes well with it. Pour off any excess acetone into a disposable cup and discard.
Open the lid and add enough water to soften the top of the wood filler.
Mix the filler well using a 1/4-inch dowel. Add water as necessary until the filler is softened enough to apply.
Add a couple drops of wood glue, if the filler becomes too thin. Mix the glue in to the filler thoroughly with the dowel.