With varying changes in the moisture content of the different parts of a piece of wood comes warping. You’ve probably heard that MDF cannot warp and wonder if this is true. Well, in this piece, we’ll bring the truth to light.
Here you’ll find out the answer to questions like does MDF warp, will MDF warp when wet, or does MDF warp when painted? After providing a detailed answer to your questions, you’ll also be intimated on possible ways to prevent MDF from warping. Now to answer your question, does MDF warp? Yes, MDF warps.
Warp in MDF can arise due to differences in moisture content in a piece of wood. The exposed part will expand while the nonexposed to moisture will contract. Additionally, warping in MDF can also be caused by sagging or bending, usually caused by permanent weight on the MDF. The best way to prevent this unwanted occurrence is to apply a sealant before putting the MDF to use and store the MDF properly after production.
We’re sure you’ll love to know more about the proper way to store your MDF and how to apply sealants to MDF, so stick around; we have more information on MDF and warping.
Does MDF Stay Flat?
Absent of warping, MDF panels are incredibly flat. This quality is due to its dimensional stability and structural integrity, which gives it a comparative edge over solid wood.
Wood fibers like to move over each other when exposed to water. For natural timber, these stresses cause distortion and splitting. But MDF, unlike natural timber, would bow rather than twist.
As much as this product is stable and robust, it is not infallible because it is still made from wood. MDF panels would warp if not sealed or stored correctly. If exposed to heat or water or the general outdoors, it would bend.
Does MDF Warp When Wet?
Yes, they do. Regular MDF has better moisture-resistant properties than solid wood, but it isn’t waterproof. MDF panels absorb water and other liquids like a sponge if they are not correctly sealed on all edges. Moisture-resistant MDF isn’t waterproof either. Yes, they are highly resistant to moisture. But if exposed constantly, they too will get damaged over a long time.
To protect MDF panels from moisture and prevent warping, you need to properly seal them on all sides. It is a must, especially for the very porous edges. Some trusted and tested sealing methods include paint, lacquer, sanding sealer or Varathane.
Methods of Preventing MDF from Warping When Wet
Let’s look closer at these different sealing methods
Paint Sealing the MDF
Paint sealing is the most common sealing method. It fills up the pores in the panels and leaves a smooth finish on the top. One or two coats of enamel paint will be enough after sanding off the edges and removing saw-marks from the material.
Applying Lacquer Finish
Lacquer is a hard finish, harder and thinner than paint. This allows it to penetrate deeply into the wood and dries by evaporation, leaving a solid protective seal over the wood. It can stay on for long without yellowing, unlike varnish, shellac, or polyurethane.
Employing a Sanding sealer
This type of finish is used for any uncoated wood product: bare solid wood, MDF sheets, oriented strand board, and other products. It helps to even out subsequent painting or finishing on the wood, leaving a smoother and more consistent surface.
Using Varathane
Varathane is a polyurethane finish used for furniture, cabinets, and floors. It comes in handy when other sealants are insufficient, especially for outdoor projects. Some formulas even come with UV protection; these formulas you can adopt for direct exposure to sunlight projects. It delivers an ultra-smooth surface that is easy to sand.
Other types of wood finishing include varnish, wax, and shellac.
Does MDF Warp When Painted?
No MDF will not warp when properly painted. Painting your MDF sheets protects them from warping by preventing them from absorbing moisture. However, if not done correctly, your sheets would still warp.
How to Properly Paint MDF to Prevent Warping
Here’s how to properly paint to MDF;
Step One: Apply Prep Coats
We need to also make you aware that painting without applying prep coats does more damage to MDF than you would expect. It would be frustrating to see your sheets bending even after painting them. Not to mention that it would mean your efforts and time have been wasted.
Step Two: Choose a Suitable MDF Primer
MDF accepts paint well, but it dries out quickly and unevenly. To avoid this, we recommend applying an MDF primer seal to the sheets. It seals it and allows subsequent coats of paint to dry uniformly. We advise you to avoid using water-based primers, as moisture would seep into the panels. Use alcohol-based or oil-based primers instead.
Step Three: Apply First, Second, Undercoat and Seal
Apply the first coat of primer, then give it a little sanding down. Vacuum off the dust. Dip a cloth in white spirit gently and wipe the primed surface. Apply a second coat of primer evenly over the surface. Next, apply an undercoat. After which, you can then proceed with painting and sealing.
Moisture-resistant paints are your best option. Because for as long as the paint surface remains intact, your sheets will stay safe and flat. However, any slight crack in the paint surface is enough for moisture to seep in and damage the sheets.
Be careful to adequately prime your sheets because it determines how good your result will be after painting. When painting, you must cover your sheets all over, even areas you cannot see.
For safety, when priming, always have appropriate respiratory equipment on. For the edges of the MDF panels, you can add a layer of wood filler before priming to shield against the over-absorption of both the primer and paint.
Ways to Prevent MDF From Warping
Seeing that MDF sheets are not impervious to warping, you can waterproof them or prevent exposure to water or heat. Here are four ways to prevent your MDF sheets from warping.
Store them Properly
First, you need to realize that MDF panels should be stored horizontally on dry bearers in a cool and dry place. These bearers serve as supports to lift the sheets off the ground.
Spread a sheet of polythene to protect against moisture rising off the floor for damp floors. Also, ensure the edges of all the stacked sheets align to prevent any layer from hanging over the other.
Employ Suitable Reinforcements
MDF sheets are very liable to bend under heavy loads and even their weight. Good reinforcements help provide support and prevent sagging. You can’t expect dead flat sheets unless the supporting structures you’ve built are sufficiently strong and dead flat.
For storing lightweight items such as small plants, vases and medium-weight items such as books, MDF sheets work well. But for heavier loads, additional support and framing are required.
Seal MDF before Use
Wood contains moisture naturally, which it loses by drying and so contracts. It also absorbs moisture from the environment; this causes it to swell. The swelling and shrinkage of wood sheets cause warping.
Sealing functions as a lock that prevents entry and exit of moisture and so protects your sheets from associated stresses. You can seal with a PVA glue, lacquer, or any earlier mentioned methods.
Use Moisture-Resistant Alternatives
Some MDF products have water repellents added to them during production. These are better options for use in high-humid areas such as bathrooms and kitchens.
Moisture-resistant MDF has different grades of resistance adapted for varying levels of humidity. The higher the degree, the more resistance it provides.
Here is a table showing the various levels of moisture resistance in an MDF and how this resistance is attained
| Moisture Resistance Level | Description |
| MR10 | Here the MDF is submerged for 24hrs, and the maximum swelling falls below 5.5% |
| MR30 | Here the MDF is subjected to a six-cycle aging test, but the test is an accelerated one (ASTM D 1037) |
| MR50 | Once this level is attained its an indication that the MDF has passed MR10 and MR30 |
How to Straighten a Warped MDF?
Sometimes, you can pull the sheets into shape while working on projects such as tabletops. However, scenarios, where you can’t just make do with the warped MDF, may arise. Therefore, it is sensible to remove the warp from the sheets in these cases.
If you discover early enough, you can flatten out bent MDF sheets. Here’s how:
Sandwiching
This method of straightening warped MDF may involve laying out the sheets with the flat surface down and the bent side up in between stronger pieces like plywood. Alternatively, you can also lay a polythene sheet on the ground and a flat sheet on top. You can as well choose to place some weights on the warped MDF. However, note, this may take days to straighten out.
Weighing Down the Panels
Here, all you need to do is place the sheets on strips of wood, with the bent side facing up and the flat surface down. Next, place some weights in the center of the curve. Ensure that the weights are heavy enough to flatten the sheets but not so heavy that it breaks them.
These methods have proved quite helpful. But it doesn’t always work. So, it is best to prevent rather than cure warping.
How to Repair Water-Damaged MDF Sheets?
An MDF panel can never be the same once it has absorbed water. But if your MDF panels are still intact, you can still salvage them. Here are steps to take to repair damaged MDF.
Step 1: Dry the panels thoroughly before you begin the repair operation. Please do not work on it while it is yet wet to avoid causing further damage.
Step 2: Next, you want to clear the dust off the surface with a hoover. Wipe off any hanging paint or old finish.
Step 3: Remove the distorted areas using a coarse sanding block or an orbital sander. Sand off water damage areas repeatedly until the surface becomes even.
Step 4: Now, it’s time to apply wood filler over the holes and cracks using a spatula or a blunt knife and allow it to dry completely.
Step 5: Sand the surface repeatedly until it becomes smooth and even again.
Step 6: Apply a coat of sanding sealer and allow it to dry completely.
Step 7: Now, you can apply the finishing touches: primer, undercoat, paint, sealant. First, apply an oil-based primer or alcohol-based one. Also, do not use water-based primers. We recommend that you apply an undercoat after priming. Apply one or more coats of moisture-resistant paint using spray paint or brush. Finally, seal up with a sealant for solid protection.
Follow these steps, and your MDF panels will look as good as new.
Primers, Undercoats, or Sealers?
You’ve seen the terms “Primers, undercoats, and sealers” not once but twice. For those unfamiliar with them, we must introduce them briefly. First, you need to understand that they are all prep-coats. But they are different formulas used for various purposes.
As in preparation coat, a prep-coat is any coat of paint you apply to your material, preparing it for the actual painting. This step is the most important in the painting process. The overall result of your material depends on how properly you carry out the prep stage.
Primer
This is the first coat you apply to your MDF sheets or any other material. It acts as a foundation on which subsequent paint coats stick. This formula is specially designed to protect against wetting while providing excellent bonding properties.
Undercoat
An undercoat is usually applied after the primer and before the topcoat. This formula creates an even and level base on your sheets’ surface, making them ready for applying subsequent coats of paints.
Sealer
A sealer is a type of finish that shields your sheets from damage when applied to them. This formula is a thin liquid that seals off the pores of absorbent surfaces. It prevents the absorption of moisture from the environment. It also protects against moisture loss from within the wood to the exterior.
These prep-coats are known to provide top-tier protection against moisture and chemical reactions that damage wood. You can even get a 3-in-1 prep-coat formula instead of separate products. This would save you time and money. What’s more? You’d be a lot less confused.
Final Thoughts
Contrary to common opinion, MDF sheets warp. They have perfectly flat surfaces that can bend and curve if not stored properly or exposed to humid conditions. Remember preventing and Protecting your MDF sheets from warping is possible. All you need to remember is
- Store MDF horizontally in a cool, dry place
- Use adequate support and reinforcement where necessary
- Seal MDF with wax, varnish, paint, or lacquer
- Invest in moisture-resistant MDF
