Although wood putty is sometimes referred to as wood filler, wood putty and wood filler are two different compounds with two different concepts. Technically, putty should be referred to as either wood dough, or wood putty. Filler should be referred to as wood grain filler. Both compounds are distinctively different with different applications.
Wood putty is made from real wood with wood solids suspended in solvents. As the solvents dissolve, solids remain in the wood. It's used to fill small holes, dents, cracks or defects in wood that are no bigger than approximately 1/8 inch wide. Holes larger than this can be filled with putty, but there is no guarantee that the putty will hold, and the putty may become visible if the defect is larger than 1/8 inch in diameter. After wood putty dries, it is then sanded smooth. It accepts stain readily and putty can be purchased according to wood specie.
Putty is used in applications about the size of a small marble, using a putty knife or any flat blade. When fresh from the can, it has the consistency of soft clay and is smashed with authority into wood defects. When the defects are filled, the putty knife can then be used to smooth the putty while it is still pliable. Wood putty dries within a few minutes even inside the can, but can be brought back to a workable condition by adding a small amount of acetone to the can. Woodworkers, cabinetmakers and furniture builders use putty on almost every project.
Wood filler, also know as wood grain filler, is used as a finishing agent. It is applied to the wood surface to fill small grain deviations and flatten the surface of wood for a finer finish. Wood grain filler is a thick liquid substance with wood solids suspended in solvent. The filler is wiped onto the surface of the wood by hand with a cloth, letting the solids enter the pores and grain patterns of the wood. It is wiped off immediately and the remaining solvent evaporates, leaving the solids in the wood, providing a smooth base for subsequent stain or lacquer coats.
Wood grain filler is typically used on open grain woods such as oak or ash. Other hardwoods such as maple or birch do not benefit from filler because of their tight grain and closed pores, preventing solids from entering the wood. After the initial application of wood filler, the wood is then allowed to dry for about an hour and then sanded by hand with finish sandpaper, typically 180 or 220 grit or higher. This results in a glassy finish that can't be obtained without using wood filler.