During my 25 years as a cabinetmaker, I worked with MDF every single day. I can count the number of times people have told me to “use MDF instead” in exterior projects. Little do they know that I have spent over two decades working with MDF and have made the mistake of using it outside.
MDF can be used outside only when it has undergone a waterproofing treatment and is somewhat protected from the elements directly. Average MDF is vulnerable to the elements and can deteriorate when used outside. There is a separate class of MDF for exterior use, and common MDF doesn’t belong to it.
That said, MDF is in a unique spot where it is somewhat compatible with exterior use but can eventually be vulnerable. In this article, you will discover the extent to which MDF can be used in exterior projects, how good it is against water exposure, and whether it warps when exposed to water.
MDF and Exterior Use: A Brief Overview
The general consensus in the woodworking community is to keep MDF out of external projects. My experience somewhat agrees with this stance. MDF can handle some weather but needs to be really well protected.
Now let’s look at the specific factors that can shorten MDF’s lifespan outside, starting with water.
Can MDF Get Wet?
Plain untreated MDF cannot get wet with direct water exposure. This is crucial for woodworkers and project owners to know because they can mistakenly take MDF’s visibly smooth and grain-free appearance to mean that the material is waterproof.
MDF can resist water in the air, so moisture doesn’t impact it as much. But using plain MDF in projects where there is significant water exposure is imprudent.
This video shows the extent to which MDF is affected by mild water exposure:
All in all, you cannot assume MDF to have complete immunity to water. At the same time, it is not as vulnerable to water damage as untreated wood, thanks to the non-wood content in the b
Does MDF Swell When Wet?
MDF swells when wet after long periods of exposure to water, especially on its end grain but even the face absorbs water. You will notice an increase in thickness where water has gotten in and the face will start to develop a dimpled appearance.
Still, water is not safe for MDF. I have never seen MDF expand right after getting wet, but I have seen plenty of water stains and mushiness. The video below shows a man trying to prove a similar point:
Notice how there is almost no visible expansion, but the MDF deteriorates pretty quickly. This happens when standard MDF is used in an exterior environment and there is constant, heavy rainfall.
Before you continue reading more about MDF, please consider the pros and cons I’ve discovered from 25 years of working with the material.
|Pros of using MDF||Cons of using MDF|
Easier to paint
Requires painting before exterior use
It is termite-resistant
It can fade in sunlight
|Mold doesn’t grow on it||It can expand with water exposure|
It is generally cheaper than solid wood is not as strong as wood
|The MDF boards come in relatively standard sizes and can be used directly in cabinetry||Almost impossible to lodge nails and screws in it.|
|Usually very flat and even||Sawing it can produce toxic dust|
Did you know that it is possible to screw MDF without splitting and without the need for special screws? I show you how in this guide on how to screw MDF without splitting.
Is MDF Ruined If It Gets Wet?
In general, MDF will be completely ruined if it gets thoroughly wet. The fibers expand making a patch-up job quite difficult. The once flat surface of MDF will become lumpy and is almost impossible to restore to its original condition.
Therefore, the rule of thumb is to waterproof the material before it is used in an external environment. How water ruins wood is different from how it ruins MDF. When untreated wood soaks up water, it gives organic ground for mold and fungus to propagate.
When MDF is affected by water, it gets slightly swollen and incurs water stains. As long as you dry wet MDF in time, you have nothing to worry about.
Steps to Take When MDF Gets Wet
- Remove MDF from the water-rich environment- Alternatively, you can remove the water source from around the MDF
- Use a heat source (like a blow dryer to dry the board) – Heatgun and indirect sunlight can also be used.
- Waterproof the MDF – Coat the MDF board in a waterproof sealant to avoid this in the future.
Of course, the above is only possible in very minor cases of MDF getting exposed to water.
How Do You Dry Wet MDF?
You dry wet MDF using a heat gun or a hair dryer. This speeds up the natural evaporation of the water, soaking up the board. One has to be careful not to bring the heat gun too close to the board because doing so can harm the adhesive holding the board together.
Again, the adhesive coming undone is technically possible but has never happened to my MDF in over 25 years of working with the material on and off. Of course, I didn’t keep a single board in the sun for two decades. But I used to leave boards outside overnight, especially when I worked with the material early on.
The only difference I noticed was that wherever the sunlight was the harshest, MDF would start fading and warping. So my advice would be to use a short-exposure heat source to dry the MDF instead of putting the material in the sun.
Why Is MDF Not Suitable for Outdoor Products?
MDF is not suitable for exterior use because it can get damaged by the environment. Natural phenomena like direct sunlight and rainfall can damage MDF. The best material for outdoor products is waterproof and isn’t easily discolored.
While MDF is termite-resistant, it is not waterproof enough to qualify as an exterior-use product. Moreover, its color starts fading if the sunlight exposure is extended. However, this applies to standard MDF only.
Exterior grade MDF exists and is waterproof and heat resistant enough to survive in an outdoor environment. Such MDF has a relatively neutral color that doesn’t degrade in the sun.
What do you do with scrap MDF? Have you considered burning it? BEWARE! I would consult my post on can you burn MDF before doing so.
Is MDF Exterior Grade?
All MDF is not exterior grade, and most Medium-Density Fibreboards are not water-resistant. Exterior grade MDF does exist, and it features denser and stronger materials. Most importantly, it is waterproof.
Waterproofing is the most important aspect of an exterior-grade MDF. Without it, even the densest block of MDF would remain vulnerable to an extent. MD in MDF stands for “medium density.” MDF can be made denser only to an extent before it can’t even be considered MDF. At that point, it is technically a wood board.
Is MDF Waterproof?
While waterproof MDF does exist, most MDF is only moisture-resistant and not waterproof. This makes the average fibreboard good enough for interior projects with no water exposure. Many woodworkers find this quite limiting.
I would recommend always finding an alternative for exterior use.
Can You Make MDF Waterproof?
You can make MDF waterproof at home by using a waterproof coating. All water-resistant sealers can be used to waterproof a Medium Density Fibreboard. To the extent that the seal doesn’t have cracks, the MDF it coats is waterproof.
The best product to use to make MDF waterproof is the Eco Advance Wood Siloxane Waterproofer because it is dedicated to waterproofing wood for exterior use. Given that MDF is already water-resistant to an extent, this siloxane coat can easily make it virtually invulnerable to the environment.
It is an eco-friendly formula, which can drive up the price a little bit. For those interested in a reasonably-priced sealant for MDF waterproofing, Gorilla Waterproof Patch & Seal Liquid is a better option.
Can You Leave MDF Outside?
You can leave MDF outside for very short periods unless the board has been sealed with waterproof varnish, sealant, or paint. Over long periods, MDF can get weakened by heat as it can make the board’s color dull.
This brings up the question of whether MDF can be left outside in the shade. I have not seen any visible negative effects of leaving MDF outside in the shade. One just needs to be mindful of the environment they’re in.
In a very humid place, leaving plain MDF outside can compromise its integrity. Once you have sufficiently waterproofed the MDF boards, you can leave them outside.
Can Moisture-Resistant MDF Be Used Outside?
Moisture-resistant MDF can be used outside at one’s own risk. Moisture resistance doesn’t equal water resistance. If it rains and the MDF is only moisture resistant, it can incur water damage. But if the boards are sealed with a waterproof coat, they will be safe.
Waterproofing MDF is good, but it is not mandatory by any stretch, especially if you don’t plan to use it in an exterior project. Treating MDF for durability isn’t as crucial as treating or finishing wood.
See my tips on if you can plan MDF.
I wouldn’t spend extra time or money sealing MDF in interior-use contexts except in kitchen cabinets. Moreover, I wouldn’t recommend using MDF in a bathroom. Both of these interior contexts create a water exposure risk.
Can I Paint MDF Outside?
You can paint MDF outside with a waterproof primer if the board is average variety and without a primer, if it is an exterior grade or waterproof. MDF board painted with sufficient waterproofing coats is okay to use outside.
If you want to be truly carefree after using MDF, you have to paint it with UV-resistant and waterproof paint. After that, the board is as durable as its build allows. Priming MDF is necessary when the material isn’t waterproof. And that’s in most contexts.
I have never accidentally bought waterproof MDF. So If you did not specifically seek exterior-grade or waterproof MDF, you should assume it is not waterproof.
Pocket holes on MDF are not suitable for outside. This exposes the interior of the MDF.
Will MDF Dry Out?
MDF will dry out if it is kept in sunlight or is spot-heated with a heat gun or a hair dryer. The MDF construction is not designed to absorb water permanently. But if water remains in the board’s body for a long enough time, it can warp the board.
In other words, speeding up the healing process is in your favor. Still, you cannot scald MDF when attempting to dry it.
Here are the methods of drying MDF, starting with the least risky and ending with the riskiest one:
- Let the water evaporate in the shade – For this, you must place the MDF outside, albeit in a place where it remains unaffected by direct sunlight.
- Use a hair dryer – Gently hover a blow drier over the wet spot. A safe distance of six inches must be maintained for this.
- Use a heat gun – You need to change the distance at which you heat up the water.
- Place it in sunlight – This can potentially dry the MDF the best, but it can also fade the color of fresh MDF. Generally, this method isn’t advisable.
MDF can be kept outside, and the material isn’t as touchy as some woodworkers make it out to be. However, direct sunlight does fade its color, and if it rains a lot in your region, you should probably switch away from MDF entirely. But if you continue using MDF, you need to get exterior grade MDF or must at least seal it with a waterproof sealant.