Frequently confused with each other, wood putty and wood filler are separate products with individual formulas and functions. Texture, differences in application, formula and curing rate makes them unalike. A third product, wood grain filler -- not to be confused with wood filler -- is unlike either product and is used to achieve a smooth finish on wood surfaces.
Wood putty is applied before sanding, staining and finishing. Woodworkers rely on wood putty to fill nail holes, small knots, cracks, splits and defects. It dries hard and can be sanded smooth. Wood filler is typically applied after the wood is finished. Hardwood flooring installers and some woodworkers rely on it to fill voids, splits and defects that otherwise might have been missed before or after finishing wood. It typically doesn't need sanding and most formulas remain slightly pliable. Wood filler products accept clear coat finishes after application. Wood putty must be dry, with a subsequent sanding before it can take a finishing product.
Wood putty typically comes in a can. It has the texture of stiff dough that begins to dry as you apply it with a putty knife or other small tool. Some brands require mixing with a hardener. Wood putty is made with solvents that resist shrinking, and can be used for some exterior purposes. Wood putty contains natural wood fibers to match specific wood species. Wood putty accepts some stains to a certain degree, and may or may not be invisible after it's sanded and stained. Wood putty is sometimes also referred to as wood dough.
Wood filler typically comes in a plastic tub. The product is colored to match popular wood species. Some formulas can be colored with powders to match stain colors. Some wood fillers remain pliable and rubbery, and are more appropriate to fill smaller voids. These types remain flexible enough to move with the wood when it expands or contracts. Other proprietary formulas continue to dry to a harder consistency. Harder formulas are more appropriate for larger voids, or when a project is going to be painted. Wood filler may be water- or solvent-based. Water-based fillers are easier to work with because they clean up faster. Solvent-based wood fillers are better for heavy-duty applications. Some solvent-based filler is formulated for exterior use, but the majority of fillers are for interior use.
Wood grain filler is used to prepare open-grained wood for final finishing. Wood grain filler has a thick, liquid consistency similar to heavy syrup that fills grain lines and pores. Wood grain filler dries hard and when sanded smooth provides a slick, flat surface for the subsequent application of lacquer, varnish or other topcoat. Wood grain filler comes in two formulas: water-based and solvent-based. Water-based is easier to use. It can be thinned and cleaned with water. Solvent-based fills deeper pores than water-based filler.