Can You Use Interior Wood Filler Outside?

Wood filler is used for fixing small cracks and holes in wood and, sometimes, replacing large pieces of missing or rotted wood. When comparing different fillers, you’ll notice that you have interior and exterior products. People often wonder whether there is any real difference between the two and whether you can use interior wood filler outside.

This article will explain what interior and exterior wood fillers are and answer any questions about how they should be used.

Can You Use Interior Wood Filler Outside?

There is a reason that interior and exterior wood fillers exist as separate products. Exterior fillers are designed to be able to withstand the elements, so they won’t be damaged by rain. They may also be more flexible so they shrink and expand without cracking when the temperature changes.

Many of these exterior fillers come as a two-part putty that needs to be mixed beforehand. The combination of the two substances creates a much harder bond that is easily able to handle harsh weather conditions.

Interior fillers, on the other hand, do not have the same properties. Although the fix will hold for a while, you will need to reapply the filler often if you don’t use a dedicated exterior product. If you are painting over the filler afterwards, this will give it some protection. However, it will still not be as durable as an exterior filler.

There are interior/exterior fillers on the market which can be used for both indoor and outdoor applications. If you are unsure, these are a good way to go. The most important thing is that you always read the guidelines on the product to determine whether it is suitable for exterior projects or not.

What Happens if You Use Interior Wood Filler Outside?

If you put interior wood filler outside, it will harden and hold for a while. But when the temperature starts changing, the filler will shrink or expand, and start to crack. In some cases, it will fall out in chunks if the size changes drastically. The rain will also start to degrade the filler and weaken the repair. In the hot sun, the filler will quickly dry out and break apart.

Interior filler won’t last long at all outdoors and you will end up making the same repairs over and over. The problem is, the damage will get worse each time as you are not fixing it properly. It’s much better to use an exterior filler, preferably a two-part mixed putty, and do the repair once.

What Makes Exterior Wood Filler Different than Interior?

A wood filler consists of sandable wood fibers in a hardening resin. When applied, the resin soaks into the pores of the wood and bonds with it, sticking the filler. The majority of interior wood fillers are made in this way.

But, there are a few key differences between interior and exterior wood fillers. As a general rule, ready-mixed wood fillers are used for interior projects while two-part products, often referred to as putty, are used for exterior projects.

Wood putty is not made from the same materials as normal wood filler. It is a combination of plastic and oil-based solvents that create a durable, pliable finish. Epoxy is a very common base for exterior two-part fillers.

The oil-based solvents help to repel water, protecting the exterior filler from the elements. Water-based interior filler doesn’t have this same protection. There are often other additives in exterior filler to make it more weather-resistant.

Interior filler is usually water-based and it dries hard. Most are easily sanded and painted, so you can make repairs and then finish over the top, hiding any trace of the filler. Some exterior wood fillers also harden so they are sandable and paintable. Wood putty, on the other hand, has a more flexible quality to it so it does not crack when temperatures fluctuate. This type of putty is not paintable, but it tends to come in a range of different wood colors, so you can get a close match.

You will often find that interior fillers have a thinner consistency and are easier to apply. This also makes them good for filling in wood pores and giving you an even finish. Putty and exterior filler, on the other hand, is much denser and sometimes more difficult to apply. This thick consistency does allow you to fill large gaps and holes. 

What are Some Good Exterior Wood Fillers?

Finding the right exterior filler is key if you want your repair to hold. There are a lot of options on the market and some are much better than others. If you have small repairs to make, the KwikWood Epoxy Putty Stick is ideal. It’s a small tube of two-part filler that is easy to mix and apply, and it dries very quickly. You won’t be able to use it for a big gap in a window frame, for example, but it’s perfect for smaller fixes.

PC Woody Wood Repair paste is a better choice for severely damaged wood. It has a thick, no tack consistency, so you can easily fill large areas without worrying about it dripping or sagging.

If you have exterior wood to repair and you are trying to choose the right filler, our list of the 10 Best Exterior Wood Fillers is a great place to start.

Final Thoughts

I wouldn’t recommend using interior filler outside. It isn’t designed to withstand the elements and the repair will soon fail. There are so many excellent exterior fillers available so if you want to do the job properly and not have to repeat it, go for exterior wood filler or putty. Ideally, get a two-part filler that gives you a more durable bond and you will only need to do the job once.