Materials to Know: Medium Density Fiberboard

MDF is the cheapest and flattest wood you can buy at local hardware stores. It’s uniform in thickness, and easy to work with. It’s no wonder that it shows up in a lot of projects. MDF stands for Medium Density Fiberboard. It’s made by pressing materials together along with some steam, typically wood, fibers and glue. This bonds the fibers very tightly. Sometimes MDF is constructed much like plywood. Thinner layers of MDF will be made. Then those layers will be laminated together under glue and steam.The laminated MDF is not as good as the monolithic kind. It tends to tear and break out along the layers, but it’s hard to tell which kind you will get.

Proper way to attatch a fastener to MDF.
Proper way to attatch a fastener to MDF.

MDF is great, but it has a few properties to watch for. First, MDF is very weak in bending and tension. It has a Modulus of Elasticity that’s about half of plywood. Due to its structure, short interlocking fibers bound together by glue and pressure, it doesn’t take a lot to cause a crack, and then, quickly, a break. If you’d like to test this, take a sheet of MDF, cut it with a knife, flip it over, and hit the sheet right behind your cut. Chances are the MDF will split surprisingly easily right at that point.

Because of the way MDF is constructed, fasteners tend to pull out of it easily. This means that you must always make sure a fastener that sees dynamic loads (say a bearing mount) goes through the MDF to the other side into a washer and bolt. MDF also tends to compress locally after a time, so even with a washer and bolt it is possible that you will see some ovaling of the holes. If you’re going to use screws, make sure they don’t experience a lot of force, also choose ones with very large threads instead of a finer pitch. Lastly, always use a pilot hole in MDF. Any particle board can split in alarming ways. For example, if you just drive a screw into MDF, it may appear to go well at first. Then it will suddenly jump back against you. This happened because the screw is compressing the fibers in front of it, causing an upward force. The only thing pressing against that force is the top layer of laminate contacting the threads. The screw then jumps out, tearing the top layer of particle board apart.

MDF fibers tend to compress over time, especially if moisture is involved.
MDF fibers tend to compress over time, especially if moisture is involved.

Moisture and Glue

The biggest issue with MDF is its tendency to absorb any and all moisture. Unless it is kept perfectly dry, it will expand and eventually disintegrate. MDF, can and will mold if left damp as well, so keep that in mind. Don’t use it in constructions that stay near food or animals.

This tendency to absorb moisture makes it difficult to glue MDF. If you are laminating two sheets together, the standard wood glue like Titebond will work. Look at the edge of the board and see if there is any curvature. If there is, face the concave surfaces of the two laminates together, and spread a light coating of glue (if you use too much glue, the MDF will swell). Press them together and weight the assembly until dry.

Another recommended adhesive for laminating MDF is a spray adhesive like 3M Super 7. These adhesives don’t adsorb into the material as much. Silicone or acrylic adhesives also do well, as they’re not really “wet” (though some silicones may have the curing agent pulled out of them). These are the adhesives you’ll find backing melamine trim and sheet for finishing MDF. If you want to glue the edge of the MDF it becomes harder. The edges tend to be more absorbent and may wick away the glue and swell. It is not a trustworthy joint.

Mechanical Joiners

If you are using a joint such as a biscuit joint or dowel, make sure to get the kind that expand when they come into contact with the moisture from the glue. These work fairly well in MDF constructions because both the dowel and the MDF expand when wetted. This forms a fairly good friction fit plus some glue bonding. Watch out for dowels and biscuits in edge joints again, as MDF splits very easily.

However, if you want a joint that counts with MDF, pick through-bolted joints. If you need a reliable right angle joint use a metal bracket with bolts and washers through the holes. There are other options too, but the rule of thumb is to keep the MDF in compression and as far away from tension as you can. Tab and slot joints and finger joints work well for this reason too.

Coating and Covering

If you need to paint MDF, prepare to spend a significant amount of your time fighting MDF’s tendency to absorb the paint. However, if you must, the most effective way with the least loss, is an oil based primer sprayed on in very light layers. This lets the paint dry and cure with minimal absorption. After a few layers the wood will be sealed and  a regular water based latex paint will be very effective.

MDF as a mold master. It works, but nowhere near as well as tooling board.
One of my MDF mold masters. It works, but nowhere near as well as tooling board. You can see how oil and resin have mercilessly absorbed into it.

I’ve tried to use MDF as masters for molds before. It can work. I had a few interesting problems at first where the MDF would absorb part A of a resin mixture but not part B. This led to a mold that looked like it was curing nicely, but would stay goopy forever. I found that an application of hairspray seemed to seal the MDF enough to apply mold release effectively, but I wouldn’t guarantee any success. Also, MDF tears as it is machined. The first few pulls from the mold will have these fibers embedded in them. There’s no real way to avoid this. Tooling board is a better choice for this.

MDF and similar particulate boards are used in industry. The typical way to finish these boards are to surround them with a layer of a completely different material. In speaker construction you’ll often find MDF wrapped with carpeting, vinyl, or leather. In cabinetry and shelving MDF is usually covered in big sheets of adhesive backed melamine.

Note, there is an MDF made with an exterior glue. This one does not absorb so much moisture. It will be easier to paint, and more difficult to glue. It is more expensive though, and may not be worth the extra cost. It might be better to use a stronger, more uniform material such as Baltic Birch Plywood at that point, unless you need a specific property MDF provides.

By Ajdonaghy2 (Photography shoot) [CC BY-SA 3.0], via Wikimedia Commons
A close up of MDF fibers By Ajdonaghy2 CC

Milling

As far as machining MDF goes, there are a few options. However, before we get to those, it’s important to note that MDF is very abrasive. The fibers used are usually not clean. There may be sand and other abrasive particulates in the blend. Your tools will dull, even carbide. On top of the abrasive properties, MDF will heat your bits up a lot. Prepare to see smoke pour out of drilling operations. Since it can’t tolerate moisture, there’s no way to cool the cutting operation, and you can pretty much kiss a bit goodbye after one job. There are some higher grades of MDF called molding grade, which are a little less abrasive, and more uniform for less heat. However, it all really still applies, and again you may not make the cost savings up.

Aside from that, everything works well on it before the tool dulls. Cutting, sanding, milling, drilling, works just fine. Watch out for blowout on the exits of your cuts. A drill bit is more likely to break out the back of the hole than drill cleanly through it. This can be combated with the standard techniques, such as placing a sacrificial wood piece behind. It is also a good idea to countersink both sides of a hole in MDF, especially if joining with screws. Make sure the cosmetic side of every cut is facing up. Don’t discount hand tools when it comes to MDF, a standard cross cut saw will tear through the stuff.

MDF can be laser cut fairly well. As far as I know most MDF doesn’t have caustic glue that would damage the laser’s mirrors, but I would watch out for the exterior grade MDF. It usually chars terribly and every edge will require a wipe down with a rag.  Otherwise prepare to get black dust on every thing. It takes a laser etch well.

MDF will remain a go to for most prototyping needs. It’s flat and easy. These day’s I mostly use decent grade Baltic Birch for the things I used to get MDF for, but every now and then I’ll still dig out a piece of the stuff for a quick project. I know there’s a ton of experience with MDF out there. I’d love to know if I got something wrong. Also, the comments on the other Materials to Know have been absolutely fantastic. If you’ve got any experience to add, please do.

Title photo By Elke Wetzig CC

71 thoughts on “Materials to Know: Medium Density Fiberboard

  1. Probably the two most important things that are missing is that the glue is deadly (it’s a well known rumour that MDF is banned in America) and the ‘sawdust’ is a nightmare.

    1. About the only thing I can think of it being good for is if you need a smooth surface and a uniform, dense material at a low cost. Its lack of strength, tendency to sag, and the way it gets used inappropriately in a lot of low end furniture that starts falling apart in a few years would convince me MDF was invented in the deepest depths of Hell… except it is too flammable, so I’m not sure how they kept a sample in their R&D lab there long enough to discover how many ways it can aggravate builders.

    2. ^^ This.
      I never did much with wood or wood-based materials until recently and I quickly decided that MDF is a PITFA, it’s not even usefully cheaper than exterior grade plywood round these parts.

      Only positive things I’ve found are it’s flat and smooth, and the very thin (~3mm) “hardboard” sheets are about the cheapest rigid sheet of something you can buy from the DIY store.

      1. Hardboard is a completely different material, using the natural lignin of the source wood instead of nasty industrial glue, almost like really thick paper. Still a pain to work with, but not as offensively dangerous. I gave away all my MDF, but I keep a little hardboard in my stash for light duty smooth surfaces, usually painted.

    3. It is great in that I can convince the fellow with the large format laser cutter to cut it on there because he doesn’t have to turn the laser up *too* high… the laser being a very expensive, limited life product, after all.
      Sure, I’d prefer to use 1/8″ plywood, but it requires a lot more power to cut, sometimes fails to cut due to the variable thickness of the glue layers that hold it together, and has a tendency to catch fire. When plywood is not on fire, it is still scorching up pretty bad.

      But I suppose everybody else here has a CNC router handy and doesn’t mind filleted corners… which of you cats is in Los Angeles or Orange County and is willing to cut my stuff for me for free? Speak up!

  2. Having wanted to prototype MDF using a laser cutter I searched the wiki. It was stated clear enough that dangerous compounds are found in the glue that I steered away. Also, using HighDensityFiber board instead wasn’t so wise. A solid plume of thick black smoke that covered the entire inside of the lasers housing and set of our safety fire alarm many times. Also stained my fingers for a week.

    TL;DR It worked, but i’m concerned for our 60k laser cutter now. I wouldn’t use HDF or MDF in a laser cutter again.

    wiki link: Formaldehyde resins are commonly used to bind together the fibres in MDF, and testing has consistently revealed that MDF products emit free formaldehyde and other volatile organic compounds that pose health risks at concentrations considered unsafe, for at least several months after manufacture.

  3. MDF is good for laser cutting, except for the smell it makes during and a little after cutting. That “slightly charred MDF” odor is too close to the smell of a burning electronic component for my liking.

  4. You forgot to mention *heavy*. I spent more time than I care to remember working with 3/4″x4’x8′ MDF in a wood shop. A day of moving and cutting those sheets is a good workout. And yes, the dust is gritty, nasty, and finely powdered. So glad to be rid of that work. Should have worn a respirator, and earplugs, and a biohazard suit, and…

  5. MDF is often used by cabinet shops in kitchens that get painted.
    It’s not as weak as it is made out to be in this article though, but that depends on the thickness of the material you are working with.
    1/8″ MDF is almost like a thick cardboard, but is more ridged. Of course 1/4″ and thinner tend to break easily when bent but do flex quite a bit prior. I would not recommend it for any structural components due to the flex alone.
    The thicker parts, usually 1/2″, 5/8″ and 3/4″ offer a good dense material that can be very rigid depending on the part geometry. This is true with any material. It has it’s uses.
    MDF comes in different grades, and buying the lesser grades can result in getting board that is less dense or may have glue and filler unevenly distributed which can cause some surprising issues when a CNC blade hits a much denser chunk of glue and filler while it’s cutting through board.
    Regardless of the MDF used, it MUST be sealed after machining.

    That said, the only benefits of MDF are usually aimed at it’s density, price, and uniform properties.
    Everything else falls short though… Ply is much stronger, particle board is lighter, and various plastics can be milled much cleaner.

    1. MDF often comes from the mill one quarter to one half inch oversized. Don’t assume that a nominal 4×8 sheet is really 4×8. Measure it and have the people at Home Depot or Lowes trim it down to your exact size on the panel saw. On the other hand the extra size can compensate for saw cuts that you make.

      1. The oversize is intentional, I’m told, so you can trim off the edge damage from handling.
        Though someone also once told me it was oversize so you can make it into tongue & groove, which made me laugh out loud.

      2. “Exact size”? Asking them to cut a panel of MDF into uniform 18″ wide portions (plus a waste portion), I’m lucky if all the pieces are within 17.75-18.25″. I don’t know how they manage to foul it up so bad, it’s not even a matter of them failing to account for blade width/offset. Now I only ask them for cuts necessary to make transport easier, and make sure all pieces are at least 0.5″ bigger than any final cut. Which I have to make at home with aluminum angle, clamps, and a handheld circular saw. I wish they’d just let me operate the panel saw, but no…

  6. The article reads as if it began life as a list – Please use longer sentences, and throw in some variety; changing the punctuation and adding in some healthy emotion goes a long way!

    In any case, thank you for the wealth of information on MDF.

  7. For the little you save on material cost, the failures of the properties and suceptability to moisture, oil, popping and simply handling means I don’t use this stuff even if it’s free. My labor is worth more than my labor + MDF. Mainy because whatever I do I’ll have to do again if I use this stuff.

  8. The trick to painting MDF is to seal it with 2 coats of shellac with a light 220 sanding between coats. After that its a snap. You can paint almost anything if you give it a good sealing coat with shellac first. If you sand down past the shellac you will have to start over. It is pretty easy to do as shellac is not a very hard finish when first applied, but it is dry in 15 minutes or less.

    I have heard that Zinssers BIN sealer is a slightly better alternative. Surprise! Shellac based.

  9. Something I like about MDF is that you know exactly what you’re getting. Pretty much all MDF is the same, no matter who makes it or where it comes from or what “wood” went into it. Plywood has different numbers of ply, or knotholes and imperfections and such. MDF is just one solid slab.

    1. That can be a curse as well as a feature

      For example, picking plywood with different plies can be a bonus.

      In general, a plywood with less plies is great if you need just a little flex, plan on forming into curves, etc. plywood with a lot of plies (some of the 3/4 sheets have like 8-10) is very rigid, easy to cut precisely, and is pretty resistant to moisture and rot buildup. (Typically an outer ply will break down and be noticeable before any structural failure occurs.)

      Different types of wood in plywood can have very important effects on resonance, stiffness, etc. that play a part in building things like speaker cabinets.

  10. Crappy it might be, but hard to beat for fast, inexpensive molds. One instrument I designed is 2 x 2.5 x 1.5 meters, and has five huge FRP (fiberglass) panels for its enclosure. The molds for those panels started life as a thick laminate of dozens of large MDF sheets, routed out by a truck-sized CNC router. The panels were hand laid up directly in the MDF. Good for a few dozen pulls, the shop told me: more than that they would make a master plug and master molds from the routed piece. (and more than a few hundred or so of those they would make an aluminum injection mold). Awesome what a good shop can do.

    I’ve used it myself for (much smaller) vacuum forms. Works great. Not as nice as renshape, but a heck of a lot cheaper.

  11. Hot glue in slots, wrapped in vinyl. Guaranteed to last a season or too.
    What’s wrong with letting paint soak in? It toughens the top layer. I let lacquer or glue soak into repairs on edges and corners first before shaping and touching up damage on customers instruments. Seal the bottom edge of such plastic covered stuff with fingered on Titebond, it will prevent shredding the finish on the otherwise unprotected edge.

  12. There’s a material called Trupan (or Ultralight MDF) that I use as a spoilboard for my ShopBot. It’s density is low enough that once you skin it (mill off the hardened top & bottom surfaces), it makes a great vacuum plenum.

    1. That’s a recommendation for a lot of milling machines with a weak-moderate vacuum hold-down table. It’s kind of creepy how light it is though. We used the same thing with a 10HP blower for an entry level 4×8 machine.
      Moving to another shop that has a more expensive vacuum, the light density stuff gets squished… so we needed the medium stuff.

    2. Is this to say that, once you skin it, it is porous enough to use for a vacuum former without needing holes? Cool. I’ll have to try that. Home Despot is just 5 minutes away.

      1. Keep in mind that the stuff HD and other big box stores sell is pretty dense stuff – unless it’s specifically labelled as “ultralight” or branded as Trupan. The HD stuff can be used, but you’ll need to take at least 1/8″ from each face using a surfacing bit.

        Another tip – you can create artificial “zones” on the vacuum plenum by using new 32gal garbage bags to cover the area not occupied by the material you’re cutting.

  13. I’ve made several woodshop jigs out of MDF, including a disc sander attachment for my lathe. It works quite nicely for that. I find that it glues fine when I use biscuits. I wrapped the edge of the sander table in 1/4 inch pine (glue and brads). The trunions were pine, but I feel like they would’ve worked just fine out of MDF. I finished the whole thing off with a couple coats of blond shellac. It’s lasted over 10 years so far.

      1. Ha, in my shop we call it ka-ka-board! On a more serious note, has anyone here experimented with epoxy/mdf composites? Epoxy has excellent capillary action that could potentially mesh well with the absorbancy of the mdf. I haven’t tried it yet but I am curious as to what extent the epoxy will cause the mdf to swell, if at all. Somewhere in this thread (can’t find it back) the possibility of doing a rough 3d milling, sealing the surface, and then doing the final milling was mentioned. Epoxy could be a deeply stabilizing influence in this scenario, or maybe not… .
        Any ideas?

  14. Seal with shellac, the miracle natural insect exudate. It is disolved in ethanol or methanol, dries very quickly so several coats is not a chore. Non-toxic (Safe for coating food by the FDA – Keep the special cake looking perfect forever and it is what keeps Christmas candies from sticking together year after year.) great sealer and paint over it with anything.

    Have a box of old 78 rpm records so scratched and chipped they will never get digitized? They are made of two blobs of shellac with a sheet of paper between. Dissolve them in methanol and – tada! – black shellac!

    1. The largest user of shellac is the pharmaceuticals industry. It’s the coating they put on pills to keep them from dissolving in your mouth. Every time you pop an ibuprofen, you’re eating bug poop.

  15. How about milling an MDF mold to almost finished, then soaking the surface with shellac, acrylic lacquer, urethane or polyester resin then allowing that to fully dry or cure? Then do the finish pass on the mold for a finer, less fuzzy surface.

    Using resin, the surface could be milled a little too far, then apply another coat of resin and mill it again so there’s no MDF fibers poking through.

    To finish the mold, coat all the other surfaces to seal out moisture.

    1. Yes but when you cut through your piece and hit that metal table top you’re going to be cussing about ruined bits/blades/chisels. When you cut into your MDF top, you shrug and say oh well. 5 years later you just swap the top for a new one when the damage becomes annoying. You also won’t think twice if you decide to screw a clamp or something to the surface because this particular project is being a pain to work on.

      The best use of MDF is as a second sacrificial top layer for working surfaces to protect your tools. It is nicely uniform, smooth, and cheap so you don’t care when you hurt it. The second best use is for woodworking jigs that you don’t intend to keep around forever.

      1. I screwed a sheet to my wordbench. It also adds a serious amount of heft to a bench (97lbs/sheet) for hand planing. I screw my midi-lathe to it as well. Really helps dampen vibration

      2. The only concern there would be hitting the metal hardware that keeps the MDF together, and having the saw blade rip it out and throw it.

        Think I would rather buy a new blade than have a 3/4 wood screw shot at my leg.

  16. MDF is on material I have zilch experience with. The local consignment auction house often would have surplus material from the local m manufactured homes plant.. I can’t recall much MDF’, a lot of particle board when they use a lot of that, but no MDF.

    1. Similar idea, particle board has larger chunks (though the terms may be used interchangeably in some parts) and has slightly better mechanical properties. However when particle board does end up rotting, it falls apart very quickly.

  17. ” Since it can’t tolerate moisture, there’s no way to cool the cutting operation, and you can pretty much kiss a bit goodbye after one job.”. Umm, what about using compressed air to cool it? If your air-system is proper, there isn’t that much moisture in it.

    What about covering the MDF in wood glue and sand it down a bit before painting? I’ve been using that technique for EVA foam (sleeping mats) for cosplays, and it works great! It should work for MDF as well (I’ve yet to try this out).

  18. Just to throw a slight alternative opinion in, it does make a great sacrificial work surface in school workshops. It’s remarkably easy to clean (not pretty mind) It doesn’t damage tools (when some idiot decides to cut the edge with tin snips) it’s pretty good at self healing, and it’s cheap enough to replace annually. It is also very good as a poor man’s stage floor. Fit tight, scrw down, then paint with copious amounts of black paint. You can then screw props,set and braces into the floor, and replace individual panels at end of life.

  19. Seriously? MDF was made to fulfil a purpose and, used correctly, does exactly what it was designed for. If you want to use it inappropriately, without the safety gear and for projects it wasn’t intended then quite obviously you’re going to come across issues.

    Use cross dowels to bolt pieces together. Much stronger than glue.

  20. There is a 100% waterproof grade of MDF that’s pretty amazing. It’s designed for signmakers, but I used it to trim my front porch. It’s been up for 12 years and still looks great. I think the brand name was MDX – pretty pricey, but cheaper than Azek and way better than PFJ pine. We tested it before putting it up – cut a scrap off and submerged it in water along with a regular piece of MDF. The MDF exploded within a few hours, the MDX was unchanged after a week. Pretty impressive.

    For interior use, I have a love-hate relationship with MDF. Love the affordability and the way it takes paint, hate the weight, the dust, and the way it kills tools.

    As for the way the edges don’t take paint the same as the faces, here’s a tip: rub a little wallboard compound into the edges before priming. Just use your finger, rub it in good, and wipe off the excess. Sand it smooth and then prime it with an oil-based primer. Here’s the important part – sand down the primer lightly prior to top coating. The primer raises the “grain” slightly, so knocking that back it important. Top coat with two coats of latex or oil, preferably with a foam roller, and the finish is so smooth you’ll think you used melamine.

  21. Use bar soap or Johnson’s paste wax to lubricate the drill and cutting bits to reduce heat buildup and wear and tear.
    I realize the topic is MDF, but liquid dish soap can be used as dirt-cheap cutting fluid when doing metal work.
    Johnson paste wax makes an excellent release agent for molds.
    Antibacterial liquid dish soap will instantly kill most insects on contact straight out of the bottle.

  22. It’s pretty obvious most of the replies here are from people who have no clue of what they’re talking about.

    They have zero personal experience using MDF and are simply parroting what they think they understood after reading it multiple times in the most reliable source of information – the internet…..

    The airborne formaldehyde MDF used to release has been significantly diminished in recent years – nowhere near what it used to be. The small amount it does release dissipates as soon as it’s cut. Once it’s finished off gassing, it’s done.

    It’ll work just fine as long as you engineer for it correctly –
    – predrill if you must screw
    – no nails
    – bolts, nuts and washers are best
    – housed joints and glue will last forever in low-stress environments

    For the poster who commented on it sagging – you are right – it does sag. It has no grain and therefore has no tensile strength.

    But it does have lots of beam strength.

    If you want MDF shelves with zero sag, any of the suggestions below will either eliminate or diminish sag –
    – add a face frame to just the front (not 100% great but it’ll work)
    – add a frame to all the edges
    – build a torsion box
    – house the shelf in dadoes in the cabinet’s back and gables and add a face frame
    I’ve done all of the above techniques. My favorites have been 6 foot shelves glued into dadoes in many book cases – the back and gables. Add a face frame on the front. Load hundreds of books for the last 16 years. Zero deflection.

    I’m not an MDF apologist. I just don’t like seeing uneducated idiots repeating a misguided trope they’ve read enough times to think it’s true with no personal knowledge.

    1. @Oliver:

      It’s good to hear that MDF manufacturers have reduced formaldehyde emissions in recent years. Do you have a reference?

      Regardless, as a hobbyist woodworker, I still hate working with the stuff. If I need that kind of surface, I will use hardboard. Because it’s thin and seems to have longer fibers, the dust seems less offensive to me. If I need more bulk in a sheet, plywood (either ordinary or “Baltic birch”) is my preference. Not saying MDF has no uses, but I don’t keep it in my shop.

  23. Currently using MDF to make low cost paneled wainscoting.Since I’m making it myself from sheets, it’s raking a while to cut and route all the rails, stiles, and panels. It will be finish painted with an airless sprayer to a semi-gloss white. IMO, all the other trim I made with MDF came out really well.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s