Even the most expert woodworker faces open cracks in joints or nail holes in their woodworking projects. Filling these blemishes is the role of wood putty or wood filler, two products that sound the same but which have important differences.
The terms wood filler and wood putty are often used interchangeably, although to expert woodworkers there are key differences between these products. Wood fillers typically consist of pulverized wood byproducts suspended in some kind of binder that dries rock hard, while wood putties are somewhat flexible materials that remain slightly pliable even after they dry. Wood putties are generally used to fill very small holes, such as the recesses where finish nails are driven below the surface of the wood.
Wood fillers are usually formulated with wood byproducts such as sawdust or wood dust suspended in a water- or petroleum-based medium. Water-based products should be used for only indoor applications, while petroleum-based fillers will usually stand up fairly well to outdoor use. These products dry very hard, but while they bond tightly to the wood, they are not true structural materials and will not improve the strength of wood joints. A wood filler is usually untinted; it is stained along with the surrounding wood after it is applied and sanded smooth.
By contrast, wood putties generally use synthetic materials such as epoxy or polyurethane, and they don't harden in the same way as wood fillers. They tend to remain somewhat flexible and are used more often to provide a flexible fill material for very small cracks or nail divots. Wood putties are often applied to finished wood and are available in many tints to match different wood finishes.
Wood fillers offer the bigger challenge for application but are also more useful during the actual construction of woodworking projects.
Water-Based vs. Petroleum-Based Wood Fillers
There are two basic types of wood filler: water-based and petroleum-based. Products like Elmer's Wood Filler are water-based and have a crumbly texture. You can identify water-based putties or fillers because the instructions call for water as the solvent for cleaning up tools.
You can identify petroleum-based wood fillers, such as Ace Wood Filler, because they will list mineral spirits, acetone, or some other chemical solvent for clean-up chores. These types of wood putties also cover and fill gaps, but they also bond wood parts together so that gaps and open joints are less likely to develop cracks over time.
Wood filler isn't a replacement for mechanical fasteners or wood glue. Wood filler is for situations where you need to visually fill a surface and provide some structural stability, but it does not reinforce a joint in the same way as metal connectors or wood glue.
Equipment / Tools
- Putty knife
- Oscillating sander
- Shop vacuum (optional)
- Wood filler
- Mineral spirits or acetone (if using petroleum-based filler)
- Sandpaper (variety of grits, from medium to very fine)
- Tack cloth
- Wood stain
Mix the Wood Filler and Apply
To apply wood filler, first, mix it up in its container with a putty knife (wood filler can separate after it sits for a while). Mix the material until it is a uniform color and texture.
Apply the Filler With the Knife
Petroleum-based filler hardens quickly, so you'll need to work fast. By contrast, water-based products are creamy and stay wet much longer.
Firmly press the wood filler deep into the crack, then scrape off the excess, making sure not to gouge the knife into the patched area.
It's tempting to load up the putty knife with lots of filler and slather it on. Inexperienced woodworkers often imagine that they can simply sand off the excess. But these products usually harden rock-solid and can be an enormous task to sand smooth. In fact, if you're filling a softwood (like pine), the filler may become stronger and harder than the wood itself, making it quite difficult to sand. Be conservative when you apply the filler to the joint or crack with the putty knife; you can always apply more if it shrinks slightly as it dries.
Smooth With a Finger
Press the wood filler deeper into the crack with your finger, then wipe off the excess from the wood. To remove the filler from your finger, quickly wipe it off with a dry cloth. If any residue remains, you can remove it with water (for water-based products), or mineral spirits (for petroleum-based wood filler).
Scrape the filler from your putty knife onto on a scrap of wood, then remove any residue off the blade using a rag and water, or mineral spirits.
Sand the Wood Filler Smooth
Give the filler at least 30 minutes to harden; a full hour is even better. True wood fillers may dry so hard that you'll have a tough time hand-sanding it. It's better to use an oscillating sander, starting with medium-grit sandpaper and following with fine 180- or 220-grit paper. Conclude by hand-sanding with 220-grit sandpaper, rubbing at the filled area in the same direction as the wood grain. This will leave faint scratches in the filler that match the pattern of the surrounding wood and will help the filler accept the stain in a more realistic manner.
Wipe With a Tack Cloth
Sanding dust left on the surface can create problems during staining or finishing. The liquid will mix with the dust and produce a lumpy, grainy surface—exactly the opposite of what you want from a nicely sanded surface. A tack cloth is essentially a sticky cheesecloth that is designed to remove fine dust from wood surfaces before finishing.
If you have a huge amount of dust, use a towel or a shop vacuum to remove the bulk of the dust, then use a tack cloth to remove the fine dust film.
Finish the Wood
Once the area is clean, the filled crack or patch is ready for stain and/or a protective finish. Staining the project will help to equalize the color differences between the filler and the surrounding wood, but don't expect the filler to be entirely invisible. Because wood filler absorbs stain differently than unfinished wood, the color is rarely a perfect match.
Expert woodworkers sometimes pre-color the wood filler before application in order to make it blend in better with the wood finish. Wood stain can be mixed into the wet filler before it is applied. This takes some experimentation since wood fillers absorb stain in a different way than does natural wood. For example, you may need to use a darker stain when tinting the filler in order to match the color you plan to use on the overall project.
Make sure to use water-based stains if you are using a water-based wood filler; solvent-based stains if you are using a petroleum-based wood filler.