Medium-density fiberboard, or MDF, is a great material for lots of different woodworking projects.
However, if you are not careful, and fail to seal the MDF board, moisture could undo all your hard work.
This is especially true if you plan to paint your project because, let’s face it—as useful as MDF is, it isn’t very pretty unless you add a layer of paint.
Painting without priming first could do irreparable damage to the wood.
The last thing you want is to spend all your time and effort crafting your wooden creation only to have it become warped as you paint.
In this post, I will discuss the importance of sealing MDF edges and how to get it done in the most efficient way.
But First, what is MDF anyway?
What is MDF Material?
Medium-density fiberboard is a pressed collection of small wood particles that didn’t make the “cut” to be used in normal lumber and wood planks.
To make MDF, all the wood leftovers are broken up into small pieces and then pressed together with waxes and resins to form solid boards.
This results in a much cheaper alternative to wood planks and uses leftovers that would otherwise be thrown out.
MDF is similar to—but should not be confused with—particleboard, which is made from sawdust.
Unlike particleboard, MDF is of higher quality, because it’s made from wood fibers.
What is MDF Used For?
MDF is an excellent material for a wide variety of woodworking projects because a single piece can usually be cut to the exact size and dimensions you need.
You also don’t have to deal with knots when you are using MDF.
Because it is such an inexpensive alternative to lumber planks, MDF is commonly used in furniture and simple woodworking projects (like for building shelves, TV stands, and speaker boxes).
Why Should You Seal the MDF?
One of the biggest issues with MDF is that it does not hold screws very well because of its makeup. MDF is also very susceptible to water damage—including the water from paints.
Because MDF is highly absorbent, even a tiny amount of moisture can cause it to warp.
This is why if you plan to paint your MDF woodwork project at all, sealing first is a must at the time your work will be near humidity or water.
I recommend that the last step before painting any MDF project should be sealing the MDF cut edges and surface. Plus, you use a high-quality wood sealant that can fix the small holes in the wood and can protect it after finishing.
This will prevent the material from getting warped if it’s near any kind of moisture source.
How Do You Seal MDF Before Painting?
Luckily, MDF sealing is a pretty straightforward process.
While you can use a few different sealers and primers to get the job done, the most important thing to keep in mind is that you want to keep the moisture to an absolute minimum.
The first thing you want to do is seal the edges of your boards because these are the most absorbent part of the wooden board.
You can seal the MDF edges with PVA glue, oil-based filling primers, or even a combination of drywall compound and primer.
Though the process is the same for either, I think PVA glue is the easiest way to seal MDF edges. All you have to do is apply an even coating onto the surface, let it dry, and then sand it down lightly.
Don’t sand too far, though; you don’t want to completely rub off the glue or primer that you just applied—you’ll have to start over again if you do.
Using a drywall compound and primer works as well, but the process is slightly longer.
First, rub the compound onto the edge evenly, wiping up any excess as needed. Then, sand it down as you would with the previous option.
Once you have a dry, evenly sanded coating, apply the primer.
After you are done with the cut edges of the board, use the same process on the other MDF surfaces.
MDF panels and crown moldings can also be sealed perfectly with polyurethane, as they can efficiently protect the wood from water damage.
If you plan to use this varnish, make sure that you use a synthetic brush and apply the topcoat evenly with smooth strokes that cover the entire wood area.
Allow the surface to dry for about 30-40 minutes and recoat if required.
With any of these techniques, choosing the best primer for MDF that will dry quickly is crucial. However, this will vary based on the climate of where you live and the humidity.
Can You Use Paste Wax On MDF Edges?
Yes, applying a coat or two of paste wax can get the job done and will make the MDF water-resistant (nearly waterproof).
In fact, I once used Minwax paste wax to finish my MDF benchtop, and it worked perfectly fine at repelling water, liquids, and glues.
Ensure you do not use any other wax (like automotive wax) and be prepared to apply a fresh coat of paste wax every year or so to the exposed edges, if any.
If you wish to make the MDF waterproof, you can also use shellac for sealing.
MDF is a kind of compressed wood material that will soak up enough of whatever is applied first. Shellac being comparatively less expensive than others, it’s an ideal choice for most carpenters and painters to cure the wood before getting it painted.
When using, you can pick poly or use de-waxed shellac that works as the best sealer for most of the MDF surfaces and edges.
Things to Keep in Mind When Sealing MDF
While PVA glue is the simplest way to seal MDF, it is vital that you use only the right type.
Some glues require mixing with water to get the right consistency—which is something you should try to avoid if possible.
a). If you opt to use PVA glue, look for acid-neutral glues, such as those commonly used with books and paper.
b). A roller is usually the best way to apply the glue, but you can also use a spray bottle if it’s designed to work with paints.
c). If you use a paint primer, avoid anything water-based, like the plague. Always go for solvent-based primers—lacquer or enamel is best.
d). Heavier bodied stain covering primers can work as well, but they are usually too thick to use if you have any fine detail in your woodwork.
The heavier primers will fill in those details you worked so hard to create.
e). If you go for the enamel paint primer, make sure to apply the coats as thinly as possible and be mindful of the color—you want one that will go well with your final paint color.
f). Lacquer primers work for sealing MDF, as well, but can sometimes create too smooth of a finish and can make it harder for your later coats of paint to adhere to the surface.
For this reason, if you plan to use lacquer, make sure to test your paints on it before you go through all the effort of sealing your MDF board edges.
The bottom line
MDF and particle board is being used for many household applications because it’s easier to work with.
Unlike regular plywood or particle board, these can be finished with less effort, and the surface looks new for a long without much maintenance.
If you plan to use medium-density fiberboard, follow the process right and seal the surface and edges well before finishing or painting.
Depending on your personal requirements, humidity, and other factors, choose the best MDF sealant that can work for you.
With the above tips in mind, properly sealing MDF for your next project will be a breeze, and you can rest easy knowing that your creation is safe from water damage.
Jack Luis is a semi-retired painter who loved painting his clients’ ideas on their walls.
He had worked as a painter for over a decade serving customers in areas such as Charleston, Mount Pleasant, Beaufort, and Georgetown, SC (South Carolina). Today in his free time, he likes to read and write about the newer techniques implemented in his profession. You may read more about him here or get in touch with him here.