How to Waterproof MDF: A Practical Guide

MDF is less expensive than other panel boards, including plywood, and has many uses. It can be used for baseboards, cabinetry, countertops, decorative panels, furniture, flooring, shelving, signage, soundproofing, underlay, and many other items. Unfortunately, most MDF isn’t waterproof, however, if you want to know how to waterproof MDF, we’re here to help!

MDF is an inexpensive engineered wood product that typically isn’t waterproof. However, you could opt for a much more expensive waterproof MDF. Alternatively, applying several coats of quality wood sealer or primer and one or two coats of waterproof paint to all surfaces is an inexpensive way to waterproof MDF.

In this guide, we’ll discuss how MDF reacts to moisture, whether it can be used in moist areas indoors or out, and if paint and other finishes will make it waterproof. We’ll provide a step-by-step guide on how to make MDF waterproof. Plus, explain how to seal the edges, where to buy moisture-resistant MDF, and identify the best way to waterproof MDF.

How to Waterproof MDF

Is MDF Waterproof?

MDF has a hardened exterior of resin adhesive and wood fiber that makes it moisture-resistant, but it isn’t waterproof. Additionally, any saw cuts or surface work will expose the interior wood fibers to moisture, which will cause them to swell. However, it is relatively easy to improve the moisture resistance to make MDF more waterproof. Whether used for interior or exterior purposes, it will require regular upkeep to prevent daily wear and exposure from damaging the protective coating which will cause the MDF to warp, bend, or deteriorate.

Can MDF Get Wet?

MDF is moisture-resistant so a little bit of moisture shouldn’t hurt it. Having said that though, immersing or sitting MDF in water for any period of time can cause it to swell, warp, and expand. Water being splashed on it and wiped off won’t damage the hardened surface, but beads of water left to dry on it can cause the finish to get fuzzy.

The fuzziness is due to the moisture being absorbed and swelling the wood fibers once the hardened adhesive is softened by the moisture. The longer moisture sits on MDF, the more likely it is to cause damage, even to moisture-resistant versions of MDF. However, there are ways to protect MDF from moisture, or you can track down a supplier of Medite Exterior MDF or Medite Tricoya Extreme MDF, which are exterior rated to last for 50-years.

Can You Use MDF Outside?

Using standard MDF outside is possible but isn’t recommended. MDF used outside needs to be fully sealed on both faces and all edges, grooves, cuts, screw holes, etc. to prevent moisture absorption and damage. That means maintaining and reapplying the sealing material once or twice a year, and sometimes more frequently depending on environmental conditions and use or abuse. There are, however, moisture-resistant and even waterproof MDF materials available, but they’re much more expensive than standard MDF.

Is MDF Waterproof When Painted?

MDF has a hard, smooth finish and is designed to be painted or stained, not left unfinished. Painting will help to seal the surface and protect it from moisture damage but not necessarily make it water-resistant or waterproof. It is best to use a waterproof sealer prior to painting, as the paint itself may not withstand the environmental or use conditions.

Using a waterproof sealer or primer along with waterproof paint will greatly improve the waterproof finish too. It is important to note that all edges, cuts, grooves, and holes for fasteners be sealed to keep moisture out.

Can You Make MDF Board Waterproof?

Is MDF Waterproof

MDF is typically moisture-resistant but not water-resistant. It is possible to make regular MDF more water or moisture resistant, but starting with moisture-resistant (MR) MDF may be better as it is made with moisture-resistant resins, making it more expensive. Begin by applying quality wood sealer or primer prior to finishing with moisture-resistant paint, or use stain, varnish, or lacquer to seal the wood.

However, some water-based polyurethanes and finishes won’t adhere to the surfaces, so steer clear of them. Make sure to check that the product of choice is designed for use on MDF material, you may need to dig into the manufacturer’s website for that information. If possible, choose products that state they are 100% waterproof, not merely water-resistant.

Pro Note: waterproofing will protect MDF from water and moisture. It isn’t intended to protect it from prolonged immersion in water.

1. Wood Sealant

KILZ Adhesion Primer, Interior/Exterior, 1 QuartImproving the moisture or water resistance of MDF begins with a good quality wood sealant. MDF or MR-MDF both accept paint and stain well, so using a brush and roller or sprayer works for applying most finishes. Some finishes require the MDF to be lightly sanded prior to applying the finish, so always read the manufacturer’s instructions.

There are several great wood sealants on the market, such as KILZ Adhesion for interior and exterior use. It’s a fast-drying, high-bonding, latex (water-based) primer and sealer for prepping multiple types of surfaces, including MDF.

It seals porous surfaces and bonds to most tough-to-paint materials to provide a dense, glossy finish with excellent topcoat adhesion. So, it will seal cut edges, routered or carved surfaces, and stick to the hardened surfaces of MDF. Most wood sealants are used with a quality top-coat to make a more waterproof finish.

2. Varnish

Rust-Oleum 207008 Marine Spar Varnish, QuartVarnish is commonly used to seal wood and will penetrate porous surfaces to make them more waterproof. Varnish is a mixture of solvents, resins, and drying oils that produce a scratch and waterproof topcoat. Using an exterior grade waterproof varnish will seal MDF and make it more waterproof.

Look for an oil-based exterior grade varnish such as RUST-OLEUM SPAR Varnish. It not only is waterproof but also UV, salt, and mildew resistant. This varnish contains urethane, making it more durable and flexible, so it won’t crack with expansion and contraction, or movement. It produces a glossy finish and is dry to the touch in about 2-hours, but can take up to 3 days to fully cure.

3. Shellac

1 gal Zinsser 270976 White Zinsser, B-I-N Advanced Synthetic Shellac PrimerShellac is an inexpensive way to seal wood prior to painting it and is commonly used by carpenters and painters to cover knots so they don’t show through the finish. MDF doesn’t have knots, but the shellac will bond with the surface wood fibers, making for a more waterproof finish. Once the shellac is dry, it should be covered with waterproof paint to increase protection from moisture.

Shellac is made using the dried secretions from the Lac bug and mixing it with alcohol or another solvent, and applying it to wood. The dry secretion flakes are available in various colors, so you can mix your own, or you can purchase Zinsser’s B-I-N Advanced synthetic white shellac. It has excellent adhesion, will seal wood and MDF, and prevents stains, knots and tannins from showing through. It even blocks odors from escaping.

4. Stain

1/2 pt Minwax 22760 Weathered Oak Wood Finish Oil-Based Wood StainThere are many different types of stains on the market in both water and oil-based formats. They typically are used with a topcoat to make MDF more waterproof but may be used alone for interior or decorative effects. It should be noted, though, that due to its construction, stain doesn’t penetrate deeply into MDF.

If you’re looking for a quality oil-based wood stain for MDF, check out MINWAX Weathered Oak penetrating stain wood finish. It’s easy to apply, quick-drying, and has a rich even color. Works well on MDF for cabinetry, furniture, doors, and signage. Just apply it to the wood, let it sit for 5 to 15 minutes, and wipe off any excess. Let it dry for 5 or 6 hours, and then seal it with a clear waterproof topcoat.

5. Epoxy Sealer

TotalBoat TotalBilge Epoxy Based Bilge Paint for Boat Bilges, Bulkheads, Engine Rooms and Locker Areas (Gray, Quart)Epoxy sealers provide an impenetrable waterproof seal to what they are applied to, including MDF. They are highly durable, resist scuffs, peeling, fading, and blistering, and are UV-resistant. Epoxy sealers can be used indoors or out to waterproof MDF however, a deep scratch or pinprick can allow moisture to penetrate and cause deterioration.

There are several different epoxy sealers that don’t require mixing of different components, such as Ronseal Wood Stain Epoxy latex sealant or TotalBilge Epoxy Based Bilge Paint. Both are easy to use, tough, durable, and flexible waterproof paints that seal MDF and other materials from moisture and other substances.

6. Drying Oil

Sunnyside Corporation 87332 Pure Raw Linseed Oil, QuartDrying oil is typically a vegetable-based oil that reacts with oxygen in the air to slowly harden and dry, such as linseed, poppy seed, perilla, walnut, and tung oil. If using drying oil to seal MDF, ensure the topcoat is also oil-based for better paint adherence. However, make sure the drying oil has dried prior to applying the topcoat.

There are many different drying oils on the market, and most work the same way to seal wood. One such is Sunnyside Pure Raw Linseed Oil or the faster drying Sunnyside Boiled Linseed Oil. It will penetrate to protect the wood to help waterproof MDF. It can be applied directly to untreated wood, or be added directly to stains and oil-based paints. Raw drying oils take 2 to 6 days or more to dry and cure, while the boiled linseed usually cures within 18 hours.

7. Liquid Latex

Rust-Oleum 1976730 Painter's Touch Latex Paint, Half Pint, Flat Black, 8 Fl Oz (Pack of 1)Liquid latex or latex-based paints dry quicker than oils, produce fewer VOCs, and provide a waterproof seal that adheres well to MDF and other materials. It can be used as both a sealer and a waterproof topcoat to seal and protect against moisture. Latex paints can be applied by brush, roller, and automotive sprayer.

Rust-Oleum Painter’s Touch Ultra Cover latex paint is one such product and is available in multiple colors. Used to seal MDF, it provides a durable smooth long-lasting finish that resists fading and chipping. It can be used indoors or out and usually dries to the touch in 30 to 60 minutes.

8. PVA

Elmer's E7310 Carpenter's Wood Glue Max, Interior/Exterior, 16 Ounces , TanPVA is short for polyvinyl acetate or alcohol, a synthetic, plastic polymer used for making wood glues, thin plastic wraps or coatings, and other products. Using PVA to seal MDF typically requires thinning a PVA glue with water to a max of 50:50 PVA to water, and applying it to all surfaces. Once dry, a waterproof topcoat of paint should be applied.

A variety of manufacturers produce PVA-based wood glues, such as Elmer’s Carpenter’s Wood Glue Max. Diluted with water and painted or spread over all MDF surfaces, it will provide a strong, durable seal that dries quickly. PVA should have a topcoat of waterproof paint applied for further protection.

How to Waterproof MDF

To waterproof MDF to help prevent moisture from swelling, deforming, and otherwise damaging the product, select a sealer or primer approved for use with MDF. The following steps will help waterproof MDF baseboards, panels, cabinets, furniture, doors, trim, signage, and other items.

Step 1

Select a quality brush of appropriate width for edges, grooves, and fastening holes. For large surfaces, use a quality 3/8” nap roller for even application and surface finish.

Step 2

Shake or stir the sealer or primer as per directions. If using a roller, pour an acceptable amount into a clean paint tray or one with a new liner. Dip the brush or roll the roller in the paint.

Step 3

Apply the primer or sealer to the edges, grooves, and screw holes first and ensure the exposed wood is covered. Make sure there are no drips or puddles. Use the roller in a ‘Z’ or ‘W’ pattern to apply the sealer or primer to larger surfaces, again ensuring it is evenly covered and there are no drips. Check to make sure there are no missed spots. Let dry as per the manufacturer’s instructions.

Step 4

Store brush and roller to stay moist if applying a second coat within 6 hours or clean as per sealer or primer instructions.

Step 5

Once the one or two coats of sealer or primer are dry, apply a coat or two of waterproof paint following Steps 1 through 4.

How to Seal MDF Edges

Sealing MDF edges should be done prior to installation to maximize protection from moisture. Use a quality 1” paintbrush or small edge roller to apply waterproof sealer or primer to the MDF’s edges. Once dry, lightly sand with a sanding block to smooth any fuzziness caused by fibers swelling and hardening. Clean off the dust and apply a second coat for increased protection. Sand again and apply more coats if necessary to achieve the desired finish.

Where to Buy Moisture Resistant MDF?

Moisture resistant (MR) MDF is typically available at most home improvement and lumber stores, such as Home Depot, Menards, and Lowes. Selection varies from location to location, and some products, like Medite Exterior MDF, may need to be specially ordered.

Forest Plywood, Georgia-Pacific, Windsor Plywood, Roseburg, and Weyerhaeuser, to name a few, all manufacture moisture-resistant MDF. So, there’s a good chance there’s some in a store near you.

What Is the Best Way to Make MDF Waterproof?

The best way to make MDF waterproof is to apply several coats of quality wood sealer or primer to all surfaces, followed by one or two coats of quality waterproof paint. The most effective way is to use a quality wood sealer and a waterproof latex or epoxy paint.

The cheapest way would be to apply diluted PVA glue or shellac to all surfaces and several coats of waterproof paint. As with anything, the size and location of the project and the desired effect, affect the waterproofing discissions and costs. Hopefully, you have a better understanding of how to waterproof your MDF project.

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