What The Heck Is MDF?
Posted on | October 27, 2008 |
Whenever I get a call to make a cabinet or bookcase out of MDF, also known as medium density fiberboard, I groan. Not because MDF is an inferior product, its not, it’s because the stuff is heavy. A 4 ft. X 8 ft. sheet of ¾ in. MDF can weigh as much as 95 lbs. and I’m no Hercules.
MDF is being used more and more in paint grade cabinetry. Some great advantages that it does have over plywood, besides being much cheaper, is that it is known for its uniform density through out the board and for its smooth, machinable edges. In other words you can use a router to gussy up the edges. I’ve always had to devise a clever way to hide the edges of my plywood shelving.
So what exactly is MDF? MDF is the hot dog of the lumber industry. It is a composite panel made from residual wood particles (dust, shavings, trim and chips) bonded under heat and pressure. The glue most commonly used in bonding the wood particles is a urea-formaldehyde resin, the other is a phenol-formaldehyde resin. So, if you or your potential client have concerns about off-gassing or respiratory problems you’ll need to find a product that uses a different bonding agent. The wood (referred to as cellulosic fibers by manufacturers) used to make MDF runs the spectrum, which is why it comes in so many different colors.
The urea-formaldehyde resin has a limited moisture resistance. Actually, you should never let the stuff get wet. It swells, which is why the first coat of primer should always be either oil-based or shellac-based primer, never latex.
The other reason I groan when having to work with MDF concerns the dust that it generates when cut. I mean the dust is as fine as talcum powder. Combine the fine dust with the now air borne formaldehyde from the resinous glue and I have to add yet another safety precaution to the mix (I always wear safety glasses and ear protection when using power tools) I now have to wear a tight-fitting respirator with a high NIOSH rating. A dust mask is not a respirator. While some companies are making formaldehyde free MDF, you still need to protect your lungs from the small particles.
Most of us probably remember the particleboard furniture and cabinetry that was popular in the 70s and 80s. You probably remember just how easy these things fell apart, too. MDF is different. It’s dense, and pretty strong, but it doesn’t go together just like wood. Building with MDF can be frustrating if you use drywall screws. Drywall screws won’t cut it, in fact the best ways to assemble MDF projects is with either Confirmat screws or a biscuit joiner and glue. And when preparing to paint MDF, remember, prime it with oil-based or shellac based primer and paint it as you see fit. But if it’s a bookcase or other shelving unit that will bear a significant amount of weight, skip the latex paint. Latex paint is too soft, oil-based paints cure into a much harder topcoat and will significantly outlast the latex in this application.
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I’m a sawdust making, tool buying, work boot wearing kind of guy. When I’m not building something useful, usually for someone else, I’m writing about it for Fine Homebuilding or This Old House. I’ve spent most of my adult life honing my craft and now I want to share what I’ve learned. From tool reviews to how-to this blog will explore a bunch of interesting stuff. So strap on a tool belt or crack a cold one and let’s have some fun.
