Sealing MDF before Painting–How to Seal MDF

Last Updated on March 23, 2023 by Ernest Godia

MDF, or medium-density fiberboard, is an inexpensive and highly versatile wood composite used in shelving, furniture, molding, cabinetry, and other applications in the building industry. 

It is easy to work with but highly susceptible to moisture due to its porosity. 

Painting is one of the most preferred ways to treat MDF when using it for a project. But since MDF is highly porous, you must seal it before painting to prevent it from sucking up the paint.  

Sealing MDF before painting also helps waterproof it to avoid swelling, warping, and cracking. 

Read along to learn how to seal MDF boards and workbenches before painting.

How to seal MDF before painting 

Treating this engineered wood product requires coating it with a suitable MDF sealer on the edges and the sides. 

MDF boards and workbenches are more porous on the edges than on the surfaces. The methods used to seal these two areas are slightly different, but the best sealer for MDF will work for both the edges and the surfaces. 

This write-up discusses five different methods of sealing MDF edges before painting and three methods of sealing MDF surfaces. 

Let’s dive right in.  

Sealing MDF edges 

Medium-density fiberboards are made by combining wood pulp with wax resin and compacting the mixture into panels under high temperatures and pressure.

This manufacturing process results in highly porous edges that you must treat with a suitable MDF sealer before painting or sealing the other parts of the panel. 

You can use any MDF sealer that solidifies on the edges. Ideal options include a drywall sealer, an acrylic paint sealer, an oil-based filling primer, PVA glue, or edging tape.

The supplies you will need 

  • Sander and sandpaper 
  • Applicator (paintbrushes, putty knife)
  • Hand gloves 
  • MDF sealer
  • Plastic sheeting or drop cloth 
  • Safety goggles 

Once you have these requirements, you can choose any of the methods below to seal your MDF panels before painting them. 

Each method involves using a product that should come with the application instructions on its label. Be sure to follow these instructions for the best results.  

Nonetheless, the guidelines provided under each method should be sufficient in most cases.  

Start by preparing your workspace in a well-ventilated area by laying a tarp or plastic sheeting to collect any spills or debris. 

Next, sand and clean the MDF board or cabinet that you want to paint. Once it is smooth and dry, use any of the following methods to seal it before painting.  

Method 1: Wood filler or putty  

Wood filler is a mixture of a sandable material, such as wood dust, and a solution of hardening resin. The sandable material is mixed into the resin solution to form a thick suspension that can soak into the fibers of wood and bond with them. 

Wood putty is a paste that you can use instead of wood filler and can bond with wood fibers. 

Both materials form a tough solid coat once dry, making them one of the best ways to seal MDF edges.  

Use a putty knife to feed the filler or putty into the edges of your panels, covering all the edges. Use the edge of the putty knife to even out the filler and create an even seal on every edge.

Once done, let the filler dry completely before sanding it down and painting over it. The drying time should be on the product label. 

Method 2: MDF edging tape 

An easier way to protect the edges of your MDF panels is to cover them with edge banding or edging tape. This product is a thin strip of veneer with an adhesive pre-glued on one side. 

As the name suggests, the edging tape is designed specifically for this purpose and is available in different sizes to match various MDF panels.  

To use one, cut it into strips matching the length and width of the MDF edges you are sealing. Apply the tape to the edges of your MDF by sticking the adhesive side to the engineered wood. Be sure to let a little section of the tape go over the edge. 

After placing the edge banding strip over the MDF edge, apply some heat to create a strong bond. We recommend ironing the tape onto the panels with the iron set to medium heat. The combination of the heat and weight of the iron should do the magic.

Use a block of wood to smooth out the adhesive veneer after ironing it to make it nice and even.

After that, gently trim off the overhang and excess tape from all around the edges to make it flush and neat. Consider smoothening it with sandpaper after cutting to complete the work. 

Method 3: PVA glue

Sealing MDF with wood glue is another simple option that you may consider. It involves applying the glue to the edges using a glue spreader, or synthetic bristled brush.

You can also use a sprayer when sealing MDF with wood glue. This method is viable if you have the PVA glue in an MDF sealer spray bottle. 

It is a straightforward way of sealing the edges because the sprayer feeds the adhesive into the porous edges with little input on your part. 

Once done, let the glue dry and lightly sand it with high-grit sandpaper. Keep a light hand when scuffing it to avoid removing the adhesive layer. 

Method 4: Using drywall compound and primer

If you prefer a drywall compound, rub a thin, even coat of it into the MDF edges and let it dry. Wipe the paste with a flat edge while wet to create an even layer across the edge.

Once it dries, sand it lightly with fine sandpaper to smoothen it before applying a primer. 

You can apply the primer with a paint roller or brush. Once the coat of primer has dried, scuff it up lightly with fine sandpaper for a smooth surface. 

Method 5: Oil-based filling primer

The filling primer works much like PVA glue. Since the product has a thin consistency, you can spray it into and on the porous edges or apply it with a synthetic bristled brush.

After covering the MDF edges with a generous amount of the primer, let it dry for the length of time recommended on the label. 

When completely dry, use fine-grit sandpaper to smoothen the coated surface. Do not apply much pressure, as that might remove the entire layer.

All said, here is a quick video on sealing MDF edges:

Sealing MDF Surfaces

Once done sealing the porous edges of your MDF, the sides are next. 

You will want to ensure you sand down the entire cabinet, workbench, or board to seal its surfaces before painting. 

The sides are far less porous than the edges but still need sealing for the best results when painting. However, the sealers used on the sides should be much thinner than that on the edges. 

Sealing MDF sides before painting helps improve adhesion and prevent problems likely to occur due to the slickness of these sections. 

And proper sanding is essential as it makes the board absorb the sealer and the paint better.  

Required tools and materials 

  • A suitable MDF sealer 
  • Applicator (MDF sealer spray, paint roller, paintbrushes)
  • Putty knife
  • Sander and sandpaper (#220 and #320 grit)
  • Tack cloth
  • Drop cloth or plastic sheeting 
  • Hand gloves 

After assembling the required tools and supplies and sanding the sides with 220-grit sandpaper, and wiping it clean, use any of the methods below to seal it. 

Method 1: Sealing MDF with PVA glue

PVA glue has a thin consistency that allows it to bond to the surfaces of your MDF boards. You can spray the glue onto the surfaces or apply it with a roller, airbrush, or spreader and let it dry.

Apply thin coats of the glue to ensure it does not form lumps on the engineered wood surface when dry. Regardless of the application method chosen, ensure you cover every inch of the sides of your MDF board and let it dry. 

Once the glue has dried for the time recommended on the label, use high-grit sandpaper to scuff it up. Keep the pressure light, so you do not remove the layer(s) applied. 

While PVA glue comes in a liquid form, it dries to form a tough coat. Scuffing up this layer will help the paint stick better.

Some PVA glues will require thinning with water before use. If you use this type of glue on MDF, the boards can absorb the solvent and swell or become warped. So we recommend acid-neutral PVA glues, which are diluted already and ready to use. 

Alternatively, consider the other methods below. 

Method 2: Sealing MDF with spray-on lacquer 

Spray-on lacquer is available in clear and colored varieties, and both types can make an excellent primer for your MDF boards. 

Lacquer primers come in different formulations. Some types are formulated to dry with glossy sheens that create adhesion problems when used as primers.

So ensure you choose a product designed for exterior finishes to guarantee better adhesion. More importantly, read the product label or check with the supplier to confirm if it will work with your chosen paint.

Once you have the spray-on lacquer primer, set up your work area in a well-ventilated area away from heat sources.  

Once you have the right setting, follow the application instructions on the label to seal your MDF panels with spray-on lacquer.  

Method 3: Sealing MDF with a solvent-based paint primer 

Paint primers are an excellent way to seal your MDF surfaces because they are designed to improve adhesion for paints. 

However, not all paint primer types will cut it. Avoid water-based primers since they can damage the MDF panels by causing swelling, warping, and cracking due to moisture absorption.

Instead, go for solvent-based primers designed for use on MDF. Like lacquer-based primers, enamel primers do a great job sealing MDF surfaces. However, consider applying thin coats to avoid runs and messy drips. 

Also, you want to pay attention to the paint color you intend to use. The idea is to choose a primer color that will complement or work well with the paint color.

Method 4. Sealing MDF with shellac

You probably know shellac as a sealant used as a top coat. However, it also works well as a sealer for priming MDF surfaces and preventing bleeding. You can paint over it since it does not cause adhesion problems.

The best way to apply shellac is using a sprayer. It will dry to the touch in roughly 10 to 15 minutes. After that, consider sanding it lightly with fine-grit sandpaper before applying your selected paint over it for a smooth finish.

Method 5: Sealing MDF with sanding sealer

Sanding sealer is a clear-colored liquid primer or base finish applied to bare wood before applying a topcoat.

The product comes with its application instructions, but here is a summary of what you must do. Ensure you stir the product gently in its container to mix the contents before using it. 

Next, pour the sealer into a lined paint tray and use a roller to scoop it, and apply it onto the MDF surface.

After covering the entire surface with an even coat of sanding sealer, let it dry for about an hour and sand it to smoothen the surface. 

You can expect the sanding sealer to raise the MDF fibers, so it is crucial to sand it well with 220 grit sandpaper to smooth it out. 

Keep the pressure gentle when sanding so that you don’t sand through the layer of sanding sealer.

How to seal MDF from moisture

To seal MDF from moisture, consider coating it with a high-quality wood sealant, stain, varnish, or moisture-resistant paint. Do not seal MDF from moisture using water-based polyurethane, as this will likely not bind readily to the surface of your MDF panels.

What is the best sealer for MDF?

The best sealer for MDF is a solvent-based paint primer such as Zinsser B-I-N Shellac-Base Oil Primer

sealing MDF with Zinsser B-I-N primer

This product is formulated specifically to seal the porous MDF surface and promote adhesion of the paint applied over it. The primer creates a barrier that blocks liquids and stains between the MDF surface and the environment around it.  

It is an all-around primer and sealer, making it an ideal base coat for MDF panels and cabinets. The primer dries pretty fast, allowing you to recoat or paint over it in approximately one hour. 

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FAQs

Do you need to seal MDF before painting?

Yes, sealing MDF with an MDF primer before painting is crucial. The panels are highly porous around the edges and would suck up incredible amounts of paint, leading to swelling and distortion. The base coat helps prevent such distortion and guarantees the desired results. 

sealing MDF before painting

How do you prep MDF for painting?

To prep MDF for painting, sand it lightly with fine-grade sandpaper (120-220 grit) to improve adhesion. The panels are typically very smooth on the sides, so you must scuff them up to make the finish stick well. 

Can you seal MDF with varnish?

Yes, you can use varnish as a topcoat for MDF and not as a base coat. When sealing MDF with varnish to waterproof it, consider applying 2 to 3 coats of the sealant using a paint roller or brush in the direction of the fibers.

Varnish can be an excellent sealant, especially if you choose to stain your MDF instead of painting it. Read our article on staining MDF.

Sealing MDF before Painting: Recap 

Painting is a popular way of treating and waterproofing MDF cabinets and panels, but the process is not straightforward. Since MDF boards are porous, painting them directly can result in a swollen, rough, or distorted surface texture.

The MDF edges can also siphon up incredible amounts of paint, which can be wasteful and problematic.

Therefore, always sand your MDF lightly and then seal it with a suitable MDF sealer before painting it to achieve a smooth finish.

This post has provided five methods of sealing MDF edges and another five for the surfaces, so you can choose your preferred method. 

We hope this helps you out. Leave a comment to let us know your thoughts or experience.  

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