Medium-density fiberboard, or MDF, is a great material for lots of different woodworking projects.
If you are not careful, however, and fail to seal the MDF, 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.
What is MDF Material?
First, what is MDF anyway?
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 makes use of leftovers that would otherwise be thrown out.
MDF is similar to—but should not be confused with—particleboard, which is made from sawdust.
MDF is of higher quality than particleboard because it is made from wood fibers.
What is MDF Used For?
MDF is great 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. On the downside, because of its makeup, MDF does not hold screws very well.
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).
One of the biggest issues with MDF, however, is that it is very susceptible to water damage—including the water from paints.
This is why it is so important that the last step before painting any MDF project should be sealing it properly.
Why Should You Seal MDF?
It is important to seal the MDF cut edges and surface, especially at the time your work will be near humidity or water.
MDF is highly absorbent, so even a tiny amount of moisture can cause it to warp.
If you plan to paint your MDF woodwork project at all, sealing first is a must.
Plus you will need to make sure that you use a high-quality wood sealant that can fix the small holes in wood and can protect it after finishing.
This will also avoid the material getting warped if it’s near to any kind of moisture source.
How Do You Seal MDF before Painting?
Luckily, MDF sealing is a pretty straightforward and easy 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. The edges are the most absorbent part of the wood.
You can seal the MDF edges with PVA glue, oil-based filling primers, or even a combination of drywall compound and primer.
PVA glue and oil-based filling primers are the easiest ways to seal MDF edges. The process is the same for either.
All you have to do is apply an even coating onto the surface, let it dry, and then sand it down.
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, you have to 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, you can apply the primer.
Once you have the cut edges of the board done, you can use the same process on the other MDF surfaces.
MDF panels and crown moldings can also be sealed perfectly with polyurethane. It 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 top coat evenly with smooth strokes that cover entire wood area.
Allow the surface to dry for about 30-40 minutes and recoat if required.
With any of these techniques, it is important to choose the best primer for MDF that will dry quickly.
However, this will vary based on the climate of where you live and the humidity.
Because of this, using PVA glue for MDF material is typically the easiest way to go.
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Can You Use Paste Wax On MDF?
Yes, applying a coat or two of paste wax can get job done.
I have once used Minwax paste wax to finish my MDF bench-top and it worked perfectly fine at repelling water, liquids and glues.
Make sure that you do not use any other wax (like an automotive wax). Also, apply a fresh coat of paste wax every year or so, to keep up the finish.
If you wish to make the MDF kind of waterproof you can also use shellac for sealing.
MDF is a kind of material that will soak up enough of whatever is applied first.
As shellac is comparatively less expensive than others, it’s an ideal choice for most of the 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 important to use the right type.
Some glues require mixing with water to get the right consistency—which is something you should try to avoid if possible.If you opt to use PVA glue, look for acid-neutral glues such as the kinds commonly used with books and paper. A roller is usually the best way to apply the glue, but you can use a spray bottle as well if it is designed to work with paints. If you use a paint primer, avoid anything water-based like the plague. Always go for solvent-based primers—lacquer or enamel is best. 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.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 color of the paint. Lacquer primers work for sealing, 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 first before you go through all the effort of sealing your MDF.
With these 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.
The bottom line
MDF is being used for many household applications due to the fact that it is easier to work with.
Unlike regular plywood or a particle boar these can be finished with less efforts and the surface look new for long without much maintenance.
If you are planning to use this medium-density fiberboard for your next project, just make sure that you follow the process right and seal the surface well before finishing or painting.
Jack Luis is a semi-retired painter who loved painting his clients’ ideas on their walls. He had worked as a painter for more than a decade to serve the customers in areas such as Charleston, Mount Pleasant, Beaufort, Georgetown, SC (South Carolina). Today in his free time, he likes to read and write about the newer techniques that are being implemented in his profession. You may read more about him here or get in touch with him here.