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Do You Need to Seal MDF Before Painting?


As an ‘engineered’ product, MDF is much more stable than natural timber, and thanks to its incredibly smooth texture and lack of grain, it’s an excellent choice for painted projects.

One question people often have about MDF, though, is whether or not it needs sealing before painting, and that’s what we’ll answer in this article.


Do You Need to Seal MDF Before Painting? – The Quick Answer

We don’t recommend painting MDF without sealing, especially if you’re using water-based paint, as the MDF can absorb moisture from the paint leading to swelling or warping. MDF is also very porous, and without a sealer, it can absorb the paint leading to discoloration and blotchiness.


Read on to find out whether you can paint straight onto MDF, the various methods available for sealing MDF, and our go-to process for not only sealing MDF but for ensuring the best quality finish for whatever type of finish you decide to apply.

Can You Paint Straight Onto MDF?

A common question people often have is – do I have to seal MDF?

You can paint straight onto MDF without sealing it first, but we don’t recommend it, especially if you’re planning to use water-based paint.

Here are a couple of reasons why:

  • MDF is highly absorbent, and its ability to effectively act like a sponge and absorb moisture can lead to swelling or warping. It isn’t just water that this happens with, though, and painting directly onto MDF with water-based paints, or even watered down paints, can also lead to swelling or warping.
  • Despite being very dense, MDF is also incredibly porous, and it typically needs a sealer or primer to make it unporous. Without applying this so-called ‘prepcoat’, whatever paint you’re using can be absorbed into the MDF, leading to discoloration and blotchiness.

These issues are most likely to occur on MDF’s edges.

The faces of MDF typically come factory sanded to 150-grit or more, making them very smooth, so these can sometimes be pretty much ready to paint.

However, the cut edges are incredibly porous (much more so than the face layers), which is primarily a result of the fibrous or fuzzy edges you get with cut pieces of MDF as shown in the image below.

an image showing the fuzzy edges of MDF

©art1980/123RF.COM

Because of this, we recommend applying a sealer across both faces and the edges to help provide as consistent and uniform a coat as possible of whatever paint you decide to add next.


Sealing Versus Priming MDF: What’s the Difference?

Prepping MDF can be particularly tricky. Not just in terms of the process required, but also trying to figure out which product to use.

It doesn’t help that many people use two of the most common terms interchangeably; primers and sealers.

But, what’s the difference? And which one should you use?

Here’s a quick explanation of each one:

Primers

The main purpose of a primer is to promote adhesion.

Primers are widely used on metals where the paint typically doesn’t stick that well to the bare metal, and so a primed layer is helpful to increase the ‘stickiness’ of the topcoat of paint.

You don’t usually have adhesion issues with wood, so with MDF specifically, primers can be helpful with filling in some of the MDF’s porosity to prevent the top coat from being absorbed, minimizing discoloration or blotchiness, and providing a much more uniform finish.

Sealers

Sealers are similar to primers, but their primary function is to make the MDF’s surface non-porous.

They’re usually thicker than primers (but not always), with this helping to ‘fill’ and seal the MDF’s porous surface, particularly on its cut edges.

While better adhesion isn’t a sealer’s primary purpose, they do still increase the ‘stickiness’ between the MDF’s surface and whichever coating you’re going to apply next (similar to a primer).

Sealers are often transparent, and while some can be left uncoated, most MDF-specific sealers are designed to accept a coat of paint, given that MDF typically receives a painted finish.

As you can see, primers and sealers are very similar, and both products serve a similar purpose.

It’s not uncommon for a primer to be called a sealer and vice versa, and noting that they essentially achieve the same results, it’s probably not worth worrying too much about choosing one over the other.

Where we feel it is essential to be a bit more selective, however, is when it comes to choosing a water-based or oil-based primer or sealer.

There are a variety of MDF-specific primers and sealers available on the market, including both water-based and oil-based types.

We tend to avoid water-based primers or sealers, even ones specifically designed for use on bare MDF. It’s a personal preference, and while admittedly the risk of swelling is probably low, it’s a risk we aren’t willing to take. It’s a potential issue that you can easily avoid by using an oil-based product instead.

The only time we’d use a water-based primer or sealer is on already painted MDF (i.e. not bare MDF), with it used to improve the compatibility and adhesion of the topcoat. Using a primer or sealer for this purpose is all the more important where there could be an incompatibility between the existing paint and the new paint on your MDF.


What Can I Seal MDF With?

There are several ways to seal MDF.

Let’s look at some of the more popular options in detail:

1. Using PVA Glue

A popular option for sealing MDF is to apply a few coats of either straight PVA or a watered-down PVA mix, for example, 50:50.

There are a few disadvantages with this approach, though:

  • MDF is highly absorbent, as mentioned above, and any additional moisture from a watered-down PVA mix can lead to swelling.
  • If you apply PVA too thickly, it can be difficult, if not impossible, to sand it flat between coats, meaning that subsequent coats of paint can look rough.
  • PVA can reactivate or break down as it absorbs solvents or water from whichever paint you apply next, reversing the reduction in porosity of the MDF’s surface and potentially leading to a flaking topcoat due to poor adhesion between the PVA and the next coat of paint.

Because of this, we generally avoid using PVA to seal MDF.

We’re not saying that it isn’t effective because it certainly can be, but, in our opinion, the potential drawbacks outweigh the benefits.

2. Using a Spray-On Lacquer

Another option is to apply thin coats of either clear or colored spray-on lacquer to seal your MDF.

We’ve found that it works particularly well on MDF’s smooth face layers, but when it comes to the edges, the spray-on lacquer usually isn’t thick enough to hide the fuzziness of the MDF’s cut edges, even if you use multiple coats.

Some lacquers can also be particularly difficult to paint over. The smooth surface provided by the lacquer can also lead to poor adhesion of any subsequent layers of paint, which can cause peeling later on.

Our main issue with using spray-on lacquer is that it needs to be used with another method, particularly on the MDF’s edges, to provide the best finish, and so, it isn’t a method we typically use.

3. Using an Oil-Based Filler Primer

Sometimes referred to as a high-build primer, an oil-based filler primer will provide a sufficiently thick coating that you can sand smooth on the MDF’s fuzzy edges.

On MDF’s face surfaces, though, which are usually already pretty smooth, we’ve found that it can take a lot of extra sanding to remove the filler primer and achieve a smooth finish.

For this reason, we’d recommend using a filler primer on the edges only and using a solvent-based primer instead on the surfaces of your MDF.

Note that we’ve recommended using an oil-based primer instead of a water-based one, and that’s because water-based primers can play havoc with MDF and lead to swelling – something that you don’t get with oil-based options.

4. Using a Solvent-Based Primer Paint

As mentioned in the previous option, solvent-based primer paints can be a good option on MDF’s smooth face surfaces.

Where they aren’t as effective, though, is on MDF’s edges, as the thinner paint likely won’t be thick enough to cover any of the ‘fuzziness’ commonly found on MDF’s cut edges, even when you use multiple coats.

In this case, we’d recommend using a solvent-based primer for the surfaces of your MDF and a filler primer on the edges.

5. Using Drywall Compound Followed by Primer

Our go-to method for sealing MDF; we’ll show you how to use a combination of drywall compound and primer in the next section.

The drywall compound is applied to the edges only and sanded smooth to remove the fuzziness commonly found on MDF’s cut edges.

Once the edges are smooth, we then apply a layer of oil-based primer (avoid water-based primer when working with MDF) to the entirety of our MDF panel or sheet, including the edges and face layers.


How to Seal MDF Before Painting

As mentioned, our go-to method for sealing MDF is using a combination of drywall compound and primer.

We’ve found that this approach not only seals the MDF but also provides the best quality finish.

While the drywall compound needs to be applied by hand, you can apply the painted layers using a brush, roller, or spray system (for example, using a high-volume-low-pressure, or HVLP system, or an airless sprayer).

an image showing primer being applied to MDF using a brush

©fotohelin/123RF.COM

A spray system will provide the best overall finish, but you can achieve a good finish with a brush or roller if you take your time and spend time sanding between coats to help provide a smoother finish.

Here’s the approach we follow:

  1. First, we sand our entire sheet or panel of MDF with 150-grit sandpaper, including all edges and face surfaces.
  2. Apply a small amount of drywall compound to the MDF’s edges, using a filling knife or your finger to thin out the layer while ensuring that the edges are completely covered.
  3. Leave the drywall compound to dry completely, and then use 220-grit sandpaper to smooth the edges completely. You might need to repeat the process with more drywall compound if any imperfections remain after sanding. Use a tack cloth after sanding to remove any dust and to clean the surface.
  4. Apply your first coat of oil-based primer to the MDF’s edges and face layers using a brush, roller, or sprayer.
  5. Once the first coat of primer has dried completely, use 600-grit sandpaper to smooth the freshly primed edges and face layers, followed by a tack cloth to remove any dust and clean the surface.
  6. Apply a second coat of primer to the surface, and follow this up with sanding with 600-grit sandpaper and cleaning with a tack cloth.
  7. A third coat of primer can be applied if needed, followed by another round of light sanding with 600-grit sandpaper and cleaning using a tack cloth.
  8. Finally, apply a topcoat to finish your MDF, for example, a coat of polyurethane or a finish layer of paint if preferred. Don’t forget to check that your final coat is compatible with whatever primer you have used in the previous steps.

Final Remarks

As you can see, despite the countless benefits of using MDF, one of its biggest drawbacks is that it requires additional time and effort to seal its highly porous and absorbent faces and edges before painting.

If you take only one thing from this article, let it be that the sealer or primer layer on your MDF is arguably more important than the finish layer.

It would help if you viewed the time spent sealing and preparing your MDF for paint as an investment, something that you can’t afford to ignore if you want a good quality finish from your painted MDF.

So, go ahead and follow our step-by-step method above to both seal your MDF and provide the best foundation for whatever finish layer you want to apply, and you’ll be much more likely to achieve a professional end result.

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!

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