Wood filler has become a staple in the woodworking industry. It's used to patch small cracks, and to fill nail holes, dents, chips or gouges. Modern wood filler is made from real wood to match practically any species you're working with. Typically, wood filler is nitrocellulose-based, which means it dries fast and cleans up easily. But it also means that the wood filler often dries out in the can before you can use it all. As long as the filler hasn't hardened completely to the point of being brittle, you can bring it back to life with acetone.
Pop the lid off the wood filler. Pour just enough acetone on top of the filler to cover the surface.
Poke a stick into the filler. Make several holes in the filler so the acetone runs into them. Pour more acetone into the can until it's approximately 1/4-inch deep on top the filler. Put the lid back on and wait 15 minutes.
Remove the lid off the can. Insert the stick into the filler and begin stirring, poking and smashing it with the stick.
Stir the filler when it begins to get slightly movable. Add more acetone if the filler is still too dry to stir. Wait 15 minutes.
Stir the putty. Dig down and get all the solid putty from the bottom of the can. Work it into the mix, then continue stirring. Add more acetone until the putty has a consistency of a thick milkshake.
Be sure to read the can's contents to determine the filler make-up before thinning. Other rarely-used wood fillers include water-based formulas, which can be softened with water. Other putty formulas are gypsum-based, which you can soften with acetone or lacquer thinner.
Once putty has hardened to the point of being brittle, it cannot be softened. Avoid breathing acetone fumes.