Is MDF Waterproof? How to Seal MDF (Easy Steps)

Medium Density Fiberboard or MDF has become a popular alternative to standard wood. You can find it being used in many different settings thanks to its greater density compared to plywood. Plus, it is often cheaper and easier to find compared to many standard woods.

However, one issue with MDF is its vulnerability to moist or humid conditions. This leads to the question of whether MDF is waterproof.

Can MDF Get Wet? Is It Waterproof?

Standard MDF is not waterproof.

Medium-Density Fiberboard (MDF) is made from wood fibers that are glued together under pressure and heat. Because of this contraction, MDF is porous by nature, and hence, it can easily absorb water.

If exposed to moisture, this could lead to swelling, warping, and deterioration of MDF boards over time.

Waterproof vs. Moisture-Resistant

Keep in mind that there is a difference between moisture-resistant and waterproof.
Waterproof materials are impervious to water and can be submerged without allowing water to seep through, making them withstand prolonged and consistent exposure to water. In contrast, moisture-resistant materials can withstand humidity in the air but are not designed to be exposed to water continuously.

Exterior Grade MDF

MDF that carries MR (moisture-resistant) is external grade and you can find them on the market. You can find brands like MEDITE and Medex selling ready to use exterior grade panels.

Exterior grade MDF is more expensive and less available compared to standard MDF. Even moisture-resistant MDF is not 100% waterproof.

If you need truly waterproof material for construction, I suggest you go for marine-grade plywood.

However, specialized MDF panels like Tricoya Extreme do exist, which offer nearly waterproof characteristics suitable for external applications. The Tricoya MDF is manufactured using acetylated wood technology, a chemical treatment that enhances resistance to moisture and fungi.
The only downside is that these are expensive.

If you cannot find MR MDF or the price of Tricoya Extreme MDF is outside your budget, there are ways to waterproof standard MDF.

How to Waterproof MDF?

For those who have standard MDF, you will need to stain, varnish, or paint the material to make it waterproof.

To do that, you will need the proper tools and materials to coat the surface so that it protects the MDF. Fortunately, the stains, varnishes, and waterproof paint can all be applied using the same tools.

Tools Needed

• A high-quality stain, varnish, waterproof paint, and sealant
• Sandpaper
• Paintbrush
• Roller Tray & Roller Tray Liner
• Roller Cover

You should also have materials that cover the floor and other areas that you do not want the stain, varnish, or paint to touch. Plus, be prepared to clean the area before and after you have applied the product.

You should start by sorting the tools to ensure you have everything necessary. Make sure to follow the manufacturer’s instructions and obtain what is needed and how to use it before you get started.

One suggestion for those who have never painted, stained, or varnished MDF before is to begin with some scrap MDF materials. Practicing on them will help you become better prepared when you are ready to paint, stain, or varnish the real thing.

Once you have done that, you are ready to get started.

Step-by-Step Guide

Step 1: Preparation

Set up in an open area, preferably outside, for maximum ventilation.

If it is raining or otherwise not possible to set up outdoors, then choose a place with the best ventilation, such as a garage or shed with the door open. Add a fan to circulate the air so that any concentrations of harmful elements are dispersed so they cause no danger.

You can choose products that have no VOCs, otherwise known as volatile organic compounds, which reduce the danger. However, you will need to set up in an open area with plenty of ventilation regardless.

Step 2: Safety Gear and PPE

Wear a safety mask to protect yourself from harmful chemicals used for waterproofing.

You must wear a latex glove to protect your skin from coming in contact with the chemicals.

Also, wear clothing that is best suited for painting. Which generally means clothing that you do not care about getting any paint, stain, varnish, or sealant on the material.

Old clothing that otherwise might get tossed out is best. Otherwise, you may want to invest in a jumpsuit or similar covering to ensure that the material does not get on your clothing.

Step 3: Prepare the Tools

Start by applying the roller cover to the roller to create the most efficient spread of the material on the surface. The roller covers come in different sizes, so use one that is the best suited for the paint, stain, or varnish that is being used.

Next, place the roller tray liner inside the roller tray. This makes picking up the material easier with the roller and helps with clean-up afterwards.

Step 4: Sand the Surface

Start by sanding the MDF board using medium-grit sandpaper. This helps in preparing the surface for primer. Make sure you sand the edges as well since MDF edges are particularly absorbent.

Once you’ve finished sanding, wipe down the board with a clean, dry cloth to remove any dust or debris.

Step 5: Apply Primer Sealant

Use a paintbrush or foam roller to apply a coat of oil-based primer or an MDF-specific primer.

 Be sure that all the edges are covered and that there are no drips or build-up of sealant on the edges. Once you have covered the edges, proceed to cover the rest of the MDF board with the sealant.

Two coats of sealant are recommended for the best protection against moisture. The sealant is, in a way, a primer for the MDF board and should be used before applying the paint.

While the primer sealant is a popular choice with MDF, it is not the only product or technique you can use to prime the MDF.

Alternate Ways to Seal the Edges

The edges of the panels are often more porous, and you can use one of the following optional techniques to seal the edges.

• Bondo: This technique uses Bondo filler across the edges and along the surfaces of the MDF. This technique is designed to provide a solid seal and is perfect for smaller projects.
  However, if you have a far larger project, such as sealing many MDF boards to build a wall or something similar, then Bondo may not work for you as it dries quickly.
• Glazing Putty: This is arguably the easiest way you can seal the edges, nail holes, etc. Plus, once it dries, you can quickly sand it for a smooth finish. The only real drawback is that glazing putty is not as effective as using the Bondo technique.
  While glazing putty is not meant for waterproofing, in my experience glazing putty will work fine for most jobs as long as it is covered by paint.
• Glue & Water: This technique can be utilized as a makeshift sealant for the porous edges of MDF. It is quite cheap and effective as it only uses water and wood glue.
  Start by mixing the water and PVA wood glue in a container. You can begin to apply once you get a mixture that is wet enough to apply smoothly but sticky enough that it bonds to the surface.
  This works well for both small and large projects. However, the one drawback is that it can be difficult to see if the first coat fully covers the MDF board until it dries. So, you should apply one coat, let it fully dry, and inspect it to see if another coat is needed.

Once you have covered the MDF board with two coats, let it sit for at least five hours so that it can properly dry. When it dries, you should clean the surface of the MDF by wiping it down with a cloth. Next, you will need to prepare the tools to paint the MDF.

Step 6: Apply the Paint

It should be moisture-resistant paint. Apply the first coat to both sides of the MDF, then sand the edges until it is smooth.

Once it is dry, apply a second coat to ensure that the MDF is completely protected. This means painting the edges as well. You’ll want to leave the MDF set out so it can dry.

Step 7: Top Coat

After the paint has completely dried, apply a clear waterproof sealant like polyurethane, varnish, or lacquer. This provides an additional layer of protection for MDF meant for exterior uses.

Apply at least two coats, allowing it to dry between coats.

For interior use, you can sand the painted surface with fine-grit sandpaper and apply a second coat of paint.

The whole process will take at least two days because the sealant and paint must not only dry but cure to be fully effective. 

Tips and Tricks

While the fundamental steps for waterproofing MDF are generally the same, specific scenarios and environmental conditions may necessitate extra care.

Following are some tips to help you achieve the better results:

• Choose the Right Primer for the Job: Not all primers are created equal. Make sure to select a primer specifically designed for your application. For instance, a stain-blocking primer can be an excellent choice for bathroom MDF projects, where moisture and mildew are concerns.
• Ventilation Matters: Good ventilation is crucial, not just for your safety but for the effectiveness of the waterproofing process. Proper air circulation allows for quicker and more uniform drying, reducing the chance of imperfections and flaws.
• Test Before You Apply:
  Before you start applying any primer, paint, or sealant to your main MDF board, consider doing a patch test on a small, inconspicuous piece of MDF. This will help you see the final result of your waterproofing technique without risking your work.
• Edge Sealing:
  MDF edges are particularly prone to moisture absorption. Consider using wood filler or specialized edge sealer for added protection. Applying multiple coats to these vulnerable areas can offer an additional layer of security against warping and swelling.
• Top Coat: After applying the first coat, let it dry and carefully examine the MDF board. You should always apply a second coat, even if you do not see any flaws or openings. An additional coat can provide greater protection for longer periods.
  Sanding with fine-grit paper between the coats is the secret that will give you a smooth, professional-grade surface.

Wrapping It Up

By following the process, you can seal and waterproof standard MDF boards relatively quickly and easily.

Remember that while these waterproofing techniques are good enough for using MDF in humid conditions, I strongly recommend using marine plywood or the special Tricoya extreme MDF for exterior use.