Wood filler and wood putty are popular products used to fill holes, cracks, and gaps that exist in a wood surface, and after it’s applied the wood will not only be visually appealing but also stronger.
But is wood filler waterproof?
I learned the answer to this question the hard way long before writing this article.
Is Wood Filler Waterproof?
No, wood filler is not waterproof. However, its qualities collectively make it water-resistant.
Wood filler is what you get when you combine a sandable material with a hardening resin. Wood filler can repair wood surfaces by soaking into the wood fibers and bonding with them.
For a long time, woodworkers would combine wood dust with varnish, lacquer, or glue to make wood filler. Nowadays, however, woodworkers use wood fibers, attapulgite, gypsum, or limestone with a hardening resin (solvent- or water-based) to make wood filler.
Wood filler can be sanded and stained once it’s had enough time to cure to give you the appearance you’re after.
Kinds of Wood Filler
An effective water-resistant wood filler is one that can fill gaps, cracks, and holes while also being able to withstand frequent and sustained moisture exposure.
Epoxy is a sought-after wood filler because it’s moisture-resistant and strong. It’s also chemical-resistant, which is why it’s used on wood surfaces that are frequently exposed to harsh chemicals.
There’s also polyurethane wood filler, which isn’t as expensive as the epoxy version but just as strong.
Then there’s latex wood filler, which is the cheapest and most commonly used wood filler. You might think that latex wood filler’s lower price implies less than stellar results, but the truth is this wood filler is more resilient than you’d think.
There’s also wood grain filler, which—as its name implies—is used to fill in wood grain and make the material stronger overall. This will need to be sanded before finish can be applied.
You can check out some of my favorite wood fillers here.
Exterior vs Interior Wood Filler
Whether you’re repairing wood inside or outside, you can use wood filler. That said, you should use a wood filler that corresponds with the job you have.
So if you need wood filler to fill holes in your dining room table, opt for an interior wood filler. And if you need to start repairing cracks in your wood shed, choose a wood filler that’s designed for outdoor use.
For example, this Timbermate wood filler is specifically designed for interior use.
Is Wood Putty Waterproof?
No, just like wood filler, wood putty is not waterproof. However, it’s stain-resistant, weatherproof, peel-resistant, and crack-resistant. Because of these qualities and others, wood putty has a variety of exterior applications.
For example, if your deck’s surface is rife with gouges, nicks, nail holes, and other blemishes, you can use wood putty to fill holes, repair cracks, and make your deck more durable overall.
Kinds of Wood Putty
Wood putty is made of wood dust and a binder, usually resin, latex, or epoxy. The binder used in large part determines how durable and long-lasting the wood putty is.
For example, wood putty that’s made with a epoxy binder lasts longer than latex-bound wood putty.
There are many kinds of wood putty, including paste, liquid, and pre-mixed wood putty. Paste wood putties are the most popular but any of them will get the job done.
Exterior Wood Putty
Exterior wood putty is usually pressure-treated so it can hold up better against the elements.
Once applied with a putty knife, exterior wood putty will expand and contract in accordance with temperature fluctuations, just like how the wood surface does.
Tips for Using Exterior Wood Putty
It’s best to keep wood putty away from direct sunlight. If it’s regularly exposed to direct sunlight for a prolonged period, it can break down quicker and fail to protect the wood surface it’s been applied to.
And since it’s water-resistant and not waterproof, it works best if it’s not deluged by a high volume of rain consistently.
Wood putty can be painted over easily; all you have to do is wait for it to dry. And once paint has been applied, you won’t have to worry about the wood putty shrinking.
Making Wood Putty Waterproof
One thing you can do to make wood putty extra moisture-resistant is to use a sealant on it after it’s been applied.
Priming the wood surface before you put the wood putty on can also go a long way in keeping the putty crack-, peel-, and flake-resistant. Primer will also act as a wood glue, helping the wood putty adhere to the surface.
Can You Waterproof Wood Filler?
Without alterations, wood filler is moisture-resistant—not waterproof. But you can waterproof wood filler by applying a seal to it.
Waterproofing wood filler makes it last longer. Plus, when wood filler is strengthened, the surface it’s applied to becomes stronger by extension.
And applying a seal to wood filler will not stop you from staining or applying a protective coat of polyurethane.
How Long Does Wood Filler Last Outside?
On average, wood filler stays in good condition for about 2 to 3 years after it’s applied, and then it begins to break down.
Factors That Determine Wood Filler’s Lifespan
How long wood filler lasts outside depends in large part on how it was applied and what it’s made of.
The conditions wood filler is subjected to routinely also play a role, so if your wood filler is constantly getting damaged or worn, it’s likely it won’t last for decades.
Despite being strong in many ways, wood filler is not UV-resistant, which means if it’s constantly exposed to direct sunlight and heavy rain, it can break down quickly, possibly just a few months after application.
Tips for Using Wood Filler
Use Sawdust-Based or Wood Fiber Wood Filler
Using wood filler that’s made of sawdust or wood fibers can ensure a smoother, more attractive finished product. The wood fillers that are more mineral-heavy don’t sand as easy, nor do they blend with natural wood all that well.
That said, if you’re going to be filling many small holes, you may want to opt for a mineral-based wood filler, as these are cheaper yet still quite effective.
Let the Wood Filler Cure
Using wood filler to fill holes in wood can be effective, provided you give the filler enough time to cure. Most wood fillers cure in two to three weeks but it can vary depending on the type.
The curing process is essentially a chemical reaction that solidifies the wood filler, and after the curing process is complete is when wood filler is most weather resistant.
Use Epoxy Wood Filler
Using epoxy wood filler is ideal if you have the budget to do so. Sanding this wood filler is easy, it has necessary adhesive properties, and it takes years for epoxy fillers to shrink or break down.
Whether you want the finished surface to be stained or coated with paint, you can count on epoxy wood filler to be smooth and visually appealing afterward. You can also read to learn if wood filler is stainable for more info!
In the end, both wood filler and wood putty are moisture-resistant, and both can be waterproofed using sealant.
Which option is better for your project depends on a variety of factors, but in general both wood filler and wood putty do a good job at filling holes in finished surfaces.