How Do I Stop MDF Warping?
From being used for furniture and cabinetry to providing soundproofing or fireproofing, medium-density fiberboard, or MDF, is one of the most versatile and widely used materials available at the local home improvement store or lumberyard.
Unfortunately, despite being an ‘engineered’ product designed to excel in many applications, MDF is not without its disadvantages.
Warping, or bowing, is a particular issue we’ve had firsthand experience of, and so we can vouch for how much of a headache it can be trying to work with an out-of-shape MDF sheet.
Thankfully, there are several steps you can take to keep your MDF flat and warp-free, and that’s what we’ll cover in this article.
Here are seven ways to stop your MDF warping:
- Store your MDF sheets horizontally where possible
- Keep your MDF clear of the ground
- Use a minimum of three supports under sheets
- Keep stacked edges flush to avoid overhang
- Keep MDF somewhere dry and well-ventilated
- Use proper framing to avoid sagging
- Apply a sealant to MDF
- Use moisture-resistant MDF
Keep on reading as we’ll show you each of these techniques in more detail. We’ll also discuss why MDF warps so easily and how you can flatten your warped MDF sheets to make them usable and to save your bank balance.
Does MDF Warp Easily?
A common misconception people have about MDF is that it cannot warp.
Yes, it’s an engineered product made to tighter tolerances than could be achieved with natural wood and is widely regarded for its dimensional stability, but it can still warp.
For simplicity, we’re going to classify a ‘warped’ MDF sheet as one that isn’t completely flat.
There are two main ways that this can occur in MDF:
1. Moisture-Related Warping
MDF is highly absorbent and will soak up any water it comes into contact with, either directly, for example, from being placed on wet ground or from the atmosphere when used in humid conditions.
Usually, changes in moisture occur in particular parts of an MDF sheet at once – for example, the bottom of the sheet may absorb moisture from the wet ground while the top is exposed to direct sunlight.
Because of this, the bottom of the sheet will expand as it absorbs moisture, and the top of the sheet will contract as the moisture level reduces, and these stresses can lead to the warping or bending of the sheet.
2. Permanent Sagging or Bending Due to Weight
One of the major disadvantages of MDF is its weight, sometimes being upwards of 60% heavier than plywood.
Because of this, MDF is prone to sagging or bending under its own weight, both when stored incorrectly or when used without adequate support.
This sagging is much more common when the MDF is used horizontally, for example, for a shelf, than when used vertically, for instance, for panelling. It’s also more common in longer spans of MDF, particularly those consisting of thinner or narrower pieces.
Over time, even the most minor sag can easily become a permanent warp whereby the MDF sheet retains the new bent shape instead of returning to flat.
In our experience, the most common and easiest way for MDF to warp is where it bends out of shape due to its weight.
We’ve experienced this permanent bending (i.e. it doesn’t return to flat) both when storing our sheets horizontally between two wooden supports (more on why you shouldn’t do this later) and when resting our MDF sheets against a wall at an angle.
When it comes to moisture-related warping, we’ve found that MDF is much less likely to bend out of shape than natural woods.
In natural woods, there are far more things that can influence warping, for example, the direction of the wood grain and which part of the tree the wood originates from (for example, wood from the center is much more likely to warp than wood taken from close to the outer bark).
MDF is made from the fibers of various hardwoods and softwoods, and so has neither of these potential weaknesses.
Another common question people often have is – does MDF warp less than plywood?
While they’re both types of engineered wood, we’ve found that MDF does warp more easily than plywood.
As discussed in our article on baltic birch plywood, plywood is made up of several layers that are glued together, with the orientation of each layer swapped.
Not only does this lead to greater strength and stability versus MDF, but it also provides better resistance to warping in wet or humid conditions, given that any movement in the wood’s alternating grain effectively cancels each other out.
How Do I Stop MDF Warping?
We’ve covered the two main ways that MDF warps; through moisture changes and permanent bending or sagging due to its weight.
There are several ways to address both of these potential issues to stop your MDF warping, including:
1. Store Your MDF Properly
As mentioned, MDF is an ‘engineered’ product (in other words, it isn’t natural), and so it can be manufactured with a much more uniform thickness and flatness than natural timber.
Despite this, and just like natural timber, it’s prone to warping or bending in the time after manufacture and before use, and we’d hazard a guess that the majority of warping issues come from the MDF not being stored correctly.
There are a few things to bear in mind when it comes to storing MDF to prevent any issues:
Store your MDF sheets horizontally where possible
You can store MDF sheets vertically, but they need to be as vertical as possible (i.e. at 90-degrees to the floor) – this can be difficult to achieve, and they can be a real hazard if they were to topple over given their weight. Because of this, we prefer storing MDF horizontally.
Keep MDF clear of the ground
Keep your MDF sheets clear of the ground using equally spaced, equally sized supports.
Use a minimum of three supports
Use at least three supports to keep your MDF clear of the ground (the more you use, the lower the risk of warping or bending, so we’d always opt for more where possible).
Use a polythene sheet on wet ground
We’d recommend placing a polythene sheet on top of or under your supports on wet floors to stop moisture soaking through into your MDF.
Keep the edges of MDF sheets flush
When stacking multiple sheets, try to keep all of the edges flush to reduce the risk of damaging the corners or edges of any overhanging sheets.
Keep your MDF somewhere dry and well ventilated
Humid conditions, particularly areas where the humidity is likely to change pretty regularly, can be very problematic for MDF given how absorbent it is.
2. Use Proper Framing/Supports
As mentioned, one of the biggest disadvantages of MDF is weight, and it is prone to warping under its own weight, especially if used without adequate support.
If you’re using MDF for a tabletop, such as a model railway baseboard, or a shelf, good framing will be vital to minimizing any warping, as mentioned above.
Reducing the distance between supports, for example, between shelf brackets, will increase the support and minimize the risk of warping.
3. Apply Sealant
During manufacture, the moisture content of MDF is usually 8 ± 3%.
A 1% change in moisture content typically results in a 0.35% change in thickness in MDF.
That’s fine where the change in moisture is consistent across the entire sheet, but usually what happens is certain areas experience a greater change in moisture, for example, the face of a sheet sitting on wet ground.
A sealer will help prevent warping in two ways:
- Firstly, and perhaps the most obvious…
It’ll stop your MDF sheet from absorbing moisture, either from direct contact with water, for example, if the sheet is set on wet ground, or moisture from the atmosphere where the sheet is subjected to humid conditions.
It’ll also stop your MDF sheet from drying out further or losing moisture. It’s the changes in moisture between various parts of the MDF, for example, where one side of the sheet dries out quicker than the other, that can cause the stresses that lead to warping.By stopping moisture escaping, you prevent these stresses occurring and reduce the overall risk of warping.
One of the most common ways to seal MDF is using PVA glue, which can either be applied directly, or mixed with water, for example, in a 50/50 mix to provide a smoother finish.
For maximum protection, the PVA should be applied in multiple thin layers to all edges and both faces of the sheet.
Once dried, you can sand the PVA back to improve the look and finish, but you should be careful not to sand completely through to the base MDF otherwise will be able to absorb moisture and dry out, risking warping.
Other options are available, too, including using an oil-based filler or even a spray-based lacquer instead of PVA as we discuss in this article: Do You Need to Seal MDF Before Painting?
4. Use Moisture-Resistant MDF
If you don’t want to go to the hassle of sealing your MDF, a good option is to use MR, or moisture resistant, MDF.
Moisture resistant MDF is specifically designed for use in humid condition, for example, in kitchens or bathrooms.
There are various levels of moisture resistance available, including:
- MR10 – Where the MDF is submerged for 24-hours with the maximum swelling being below a maximum of 5.5%
- MR30 – Where the MDF is subjected to a six-cycle accelerated aging test (ASTM D 1037)
- MD50 – The highest level available, this indicates that the MDF has passed the requirements of both MR10 and MR30
The higher the grade of MR MDF you choose, the more resistant it will be to any warping that could occur from moisture being absorbed where it comes in direct contact with water, or from changes in humidity.
How Do You Remove Warp From MDF?
If you’re wondering whether you can straighten warped MDF, the answer is yes.
It is possible to flatten MDF that has bowed or warped.
In certain situations, it might be possible to use slightly warped MDF. For example, if you’re using a sheet of MDF for a tabletop, the various fixings you use to attach it to the table’s frame should, in theory, ‘pull’ it into shape.
In some situations, though, it might be necessary to try and flatten the MDF before use.
This can be tough, and it doesn’t always work, but there are two methods we’ve used with success in the past:
For Minor Warping:
To deal with minor warping, we’d recommend sandwiched the bowed board between two flat sheets of MDF (or plywood, or whatever else you have to hand).
We’ve also had success with sandwiching the board between flat ground and a second sheet of MDF, but we’d recommend putting down a polythene sheet to prevent any moisture from getting into the MDF if the ground is wet).
Whatever option you choose, make sure your MDF sheet is facing bow side up as shown below.
You might also need to add some weight to the top sheet to provide enough downward pressure to flatten the warped MDF board.
We usually leave our MDF in this ‘sandwiched’ state for a couple of days, and we come back every so often to check on progress.
For More Severe Warping:
Place your MDF sheet on top of two supports, with the bow going upwards as shown below.
Apply some weight to the center of the MDF sheet between the supports. The weight shouldn’t be so heavy that you risk breaking the MDF, but it needs to be heavy enough to start to flatten the sheet out noticeably.
From nipped fingers to dropping weights on your feet, there are plenty of risks here, so be sure to pay attention to your safety.
Clearly, when working with thicker sheets or more severe warping, the weight you’re using will need to be increased to make a difference.
We’d recommend checking on your MDF every hour or so to gauge progress. You’ll know that it has been successful if the sheet no longer springs upwards when you remove the weight.
As you can see, despite the countless benefits of using MDF, one of its biggest drawbacks is that it warps so easily, mainly as a result of its weight.
If you take only one thing from this article, let it be that most warping or bowing issues associated with MDF come from poor storage.
It’s pretty easy to deal with moisture-specific issues. Whether you opt for applying a sealant or using moisture-resistant MDF, you can significantly reduce the risk of any warping caused by water absorption making this aspect a non-issue.
So, follow the tips above and keep your MDF stored horizontally, with three or more supports underneath, and ensure that you provide enough support or adequate framing for your MDF in your project, and you should be able to stop your MDF warping completely.
If you have any particular questions or queries that you can’t find an answer to above, drop us a comment below, and we’ll do our best to get back to you as quickly as possible.
Thanks, and good luck!
If you enjoyed this article, you’ll probably enjoy the following, too:
- Our other MDF articles
- Our other articles on woodworking woods
- Our other DIY how-to articles