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What Is Wood Filler Made Of? (All You Need to Know)

In Tips, Tools by Jamie P.

Wood filler is a handy thing to have in your toolbox, it can help with a lot of woodworking projects, especially furniture makeovers. However, it’s not suitable for all repairs, and people often make mistakes because they misunderstand what wood filler is, what it is made of, and when you should use it.

I am writing this article to help clear up any confusion so you can make sure that you always pick the right product for the job.

What is Wood Filler?

Wood filler is a substance used for repairing wood damage. When you have gaps, cracks, or missing sections of wood, you can fill them with the paste-like substance, which then hardens. Making repairs with wood filler can extend the lifespan of wooden fixtures and help you from having to replace them.

There are different types of wood filler available for interior and exterior projects. Certain brands have a thicker consistency, so they are better for filling large sections, while others are more suited to fill small nail holes or hairline cracks. You can also get wood grain filler, which fills the pores in the wood to smooth over the grain before finishing.

Many wood fillers can be painted or stained after use, so the repair blends in. Knowing how to find the right wood filler and how to use it properly will help you with all kinds of interior and exterior woodworking projects. Wood filler isn’t a magical fix, it can take some serious work to fill an area to look as if it was never damaged in the first place.

applying walnut wood filler

Elmers Walnut Wood Filler being applied on a dresser I restored. See the video here on YouTube.

What Is Wood Filler Made Of?

Wood filler has two main components; a filling substance and a hardening resin. The filling substance is usually a wood by-product like sawdust and wood fibers, which are then suspended in a hardening resin. The wood fibers and sawdust allow the filler to be painted and stained like wood, while the hardening resin gives it a solid texture when it dries.

The hardening resin can be made from a number of things including epoxy, lacquer, clay, urethane, and latex. Some fillers come ready to use right out of the tub, but others come as a two-part formula. This is because the resin does not harden on its own; you need to add a separate hardener. A small amount of the hardener is mixed into the filler before application, allowing it to dry solid.

As well as the resin and the filling substance, some wood fillers are tinted with pigments or dyes to give them different wood colors. You can also tint some fillers yourself to get the desired color.

Wood filler is not to be confused with wood putty, which is made from different materials and is used for different projects.

Can You Make Your Own Wood Filler?

Yes, if you want to save yourself some money, you can make your own wood filler. You may wonder what the point is, considering wood filler is not very expensive. Why take the time to make your own if you can just buy it? Usually, the reason is so you can get the closest match with the color. Although filler comes in different shades and can often be stained or painted, making your own filler is the most effective way to get a perfect match. It’s also a convenient option if you run out of filler halfway through a project because it doesn’t take long to make a batch.

The basic elements are the same; sawdust from wood and a binding agent, usually wood glue. You can use the same wood that you are working with to get a good color match. However, the glue will change the color slightly, especially if it is tinted. If possible, use white glue.

To make your filler, you simply need to sand a piece of wood. If you are already sanding, keep the sawdust aside. Once you have a good pile, put it into a container and start adding some glue, a little at a time. Mix it up with a wooden stirrer until you have a thick paste. Add the glue slowly, so you don’t accidentally add too much and end up with a runny consistency.

The glue hardens quickly, so get to work right away. Use a scraper or putty knife to work the homemade filler into holes and let it dry. This is ideal for small holes and cracks. However, this homemade filler is not suitable for fixing big gouges in the wood or replacing sections. For these big repairs, you will need to buy some high-quality wood filler.

walnut wood filler before and after

What’s the Difference Between Wood Filler and Wood Putty?

There is often confusion between wood filler and wood putty. In fact, many people think that they’re interchangeable terms for the same thing, but that isn’t the case. They are made from different things, and they are used in different ways.

Wood putty is made from a combination of plastic and oil-based products like linseed oil or calcium carbonate, for example. Unlike wood filler, which dries hard very quickly, wood putty takes a while to dry and it remains quite pliable. In general, wood putty is used for exterior repair jobs. The flexibility of the putty means that it won’t crack due to expansion and contraction when the temperature changes. The plastic and oil-based components make it waterproof, so it can hold up against the rain. There are exterior wood fillers available, but putty is often the better choice. Fillers are prone to cracking when used outdoors because they dry very hard.

The other major difference between the two is that putty cannot be sanded or stained. Filler contains wood fibers and sawdust, which takes stain in the same way as normal wood. It dries hard, so it is easily sanded. Putty, on the other hand, will not absorb the stain. Instead, it comes in different wood shades so you can find the closest match.

Wood filler is best for interior projects and fixing holes and cracks in furniture. It’s also good for filing in old screw and nail holes or covering the top of screws. Putty is usually best for exterior projects, especially things like decking or siding that needs a durable repair. However, epoxy wood fillers do not have the same problem with shrinking caused by temperature fluctuations, so they are suitable for use outside too.

Is Wood Filler as Strong as Wood?

Wood filler dries very hard, but it is not necessarily as strong as wood. There is a difference in strength depending on the type of filler. Epoxy tends to be the most durable, with latex fillers coming a close second. These hardening resins form a very strong bond and give long-lasting repairs to the wood.

That said, if you are replacing a section of the wood with some filler, you have to bear in mind that it is bonded to the wood. That join will always be a weak spot compared to a solid piece of wood. So, even though filler is strong, a piece of wood will always be slightly weaker once you have repaired it.

How Long Does Wood Filler Take to Dry?

Wood filler dries quickly, which is one of the reasons why it’s often the preferred method for repairs. All products vary and you should check the manufacturer’s guidelines, but they usually take around half an hour before they are touch dry. After an hour, you should have a solid finish. Always give it plenty of time before you try to sand or stain the filler or you could damage it.

Is Wood Filler Toxic?

This depends on the type of filler and the ingredients. In general, fillers are not overly toxic and you won’t get seriously ill from breathing in the fumes. That said, some of the plastic-based fillers can give off strong fumes. Any spilled product could damage plant life and be harmful to pets, so make sure you put some sheets down and clean up thoroughly if using it in the yard. Otherwise, just make sure you are in a well ventilated area and you will be fine.

What Happens if You Inhale Wood Filler?

The main danger with wood filler is inhaling the dust when you sand it. You get a lot of fine dust when you sand wood filler and this will irritate your throat and lungs if you breathe it in. It will also irritate your eyes. Inhaling it on a regular basis can lead to some long-term respiratory issues, so you need to be careful.

Always wear some goggles and a mask when you are sanding wood filler, and be careful when cleaning up the dust.

Wood Filler FAQs

Does Wood Filler Last Long When Used on Furniture?

I have used wood filler numerous times while refinishing wooden furniture. With proper preparation, the wood filler will last just as long as the original finish. Why? Well to put it simply, you are going to finish the wood filler exactly how you finish the rest of the wood. Whether you stain or paint it, you are sealing it afterward. Once sealed, and done correctly, the wood filler should match the rest of the wood unnoticed and it will be protected along with the rest of the project.

The most common wood filler I use for matching with real wood is Elmer’s wood filler, you can choose from a variety of wood type/color options. There are many wood filler brands to choose from; try testing some to find what works best for you.

Can You Use Wood Filler on a Cutting Board?

Yes, but only if you use the right product. Most average wood fillers are not food safe and could make you sick if you put them on a cutting board. However, there are food grade fillers on the market. You can also get food grade glue which you can use to make your own filler.

When choosing products, be sure to know the difference between food safe and food grade. Food safe means that it is safe for indirect contact with food, but it is not considered safe to actually prepare or eat food from. Only food grade products are fully safe for cutting boards.

Shellac is the best option for repairing cutting boards. This is a natural resin that is completely safe for food preparation. You can get shellac sticks (sometimes called lacquer sticks) which can be used to fill holes and cracks in your cutting board. To use these, you gently heat them and then rub them over the wood, filling the gap with shellac. Once it has dried, you can sand it back.

What are Some Wood Filler Alternatives?

Wood filler is the most common way to fill gaps in wood, but there are alternatives. Using your own homemade filler is an option, or you could use putty for exterior jobs.

You can also make your own putty using chalk dust. Painters whiting, a fine white dust used for thickening paint and putty is the best choice for this. It’s cheap to buy, and you can make a great filler alternative for painted woodworking projects. To make it, you simply take a pile of the white dust and then add a few drops of paint at a time. Mix it until you have the right consistency to fill small holes and cracks. The great thing about this is that you can use the same paint you will use to paint the rest of the wood, so it blends easily.

It is possible to fill larger gaps with another piece of wood, but this is an advanced job. It involves changing the shape of the gap to make it uniform and then fixing in a piece that is the right size. If you get it right, it can look seamless, but this is one probably best left to experienced woodworkers.

Can You Paint Over Wood Filler?

Yes, most wood fillers can be painted over. Once the filler has dried properly, sand it down so the surface is smooth. I use 220 grit sandpaper or higher to make it super smooth. Then, wipe it with a tack cloth. You can then paint according to the standard application instructions.

Applying a primer may be required to avoid bleeds or to avoid the area being noticed. I like to prime the surface with a spray primer such as Rustoleum spray primer, or you can use a shellac-based primer such as Zinsser BIN.

Can You Stain Over Wood Filler?

Yes, but it can be tricky to get the results you want. The filler absorbs stain differently from the rest of the wood, so those patches will probably show up lighter. Water-based stains work best over the top of the filler. If the filler can be tinted, you can also mix a small amount of stain into it before you make the repair to help you get a better color match.

Other options to match the filled area are to mix and match paints to look like the wood pattern and stain color, this can also be done with pigments too.

Can Wood Filler Be Screwed into?

Yes, most wood fillers can be screwed into. If you plan to screw into filler once it has dried, consider which type to use. Epoxy wood filler is very strong and can be screwed into without cracking. Latex fillers are also flexible enough to hold a screw. However, there are limits to consider when screwing into wood filler.

Attaching a small load to a filled piece of wood is fine. But if you fix a heavy load directly into a section of filler with a screw, it could crack and rip out. So, use your best judgment and, where possible try to screw directly into the wood and avoid the filler.

Final Thoughts

Wood filler is excellent for making small repairs, particularly on interior woodworking projects. If you are working outside, always use an epoxy or latex-based wood filler and check that it says it is suitable for outdoor use. That said, putty is usually a better choice for exterior woodworking projects.

The most important thing to remember is that wood fillers are all different and you need to do your research to find the right product for the job. Testing makes perfect! Good luck with your wood filler project.