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Working With MDF

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The following article was originally featured in our new stand issue “Built-Ins” in 2006.

At the risk of inviting the scorn of some purists, I’m here to make a bold declaration: For painted cabinetry, there is no better and more appropriate product to use than medium-density fiberboard (MDF). And virtually any species available as the face veneer of hardwood plywood is also available with an MDF core.

No material is perfect, of course, but in the case of MDF, the pros far outweigh the cons. MDF is inexpensive and extremely stable, and it’s manufactured in consistent thicknesses (unlike most plywood). In some ways, using MDF is more environmentally friendly than using solid lumber because there is an inexhaustible, renewable supply of the raw material MDF is made from—a supply that will be available for generations to come. On the downside, though, sheets of MDF are heavy, and the dust that is kicked up when you cut and rout the material can be, at best, an irritating nuisance; at worst, it can be a health hazard if your exposure is extreme.

What is MDF?
Until conducting research to prepare for this article, I knew little about MDF besides that it is a wood composite. I got my hands on industry documents that describe the contents and manufacture of MDF in terms that sound almost like language you’d expect to hear in medical school—cellulosic fibers, tracheids, pendistors, and defribrators. Put in layman’s terms, most MDF can be defined as panel products made from wood that has been pulverized into tiny fibers, heated, mixed with glue, pressed to a consistent thickness, dried, and cut to size.


The industry differentiates between three density grades of MDF: low (weighs less than 40 lb. per cu. ft.), standard (40 lb. to 50 lb. per cu. ft.), and high (more than 50 lb. per cu. ft). And like any product manufactured competitively, there is a huge variety in kind and quality, even within the standard-density grade that makes up most of the MDF made and sold. Learn more about building with MDF.

Urea formaldehyde (UF) resins have been the primary adhesive for the composite-panel industry since its inception in the years shortly after World War II. UF resins lend strength and stability to the finished panels, at a reasonable cost, but they offer only limited moisture resistance. They also emit small amounts of formaldehyde, which can irritate eyes and respiratory systems and possibly cause more severe health problems in people with extreme sensitivities to it. If you or a family member fits that category, you can buy a formaldehyde-free panel. The Medite Corporation makes Medex and Medite II—moisture-resistant and interior-grade MDF panels that emit extremely low levels of formaldehyde.

Melamine-fortified UF resins and phenol-formaldehyde (PF) resins are glues sometimes used to enhance the water resistance of the final product. Methyl-diisocyanate glues are used in panels made from agrifibers for the same reason. There is no such thing as waterproof MDF, but Medex—which was developed for nonstructural, high-moisture applications such as countertops, bathroom cabinets, baseboards, and painted windowsills—is rated as highly water resistant.

The Buyers and Specifiers Guide, published by the Composite Panel Association, lists a couple of plants in Canada that make an exterior-grade MDF using spruce and pine as the raw material.

For surfaces that will be painted, MDF has no equal
For painted cabinets, furniture, wall paneling, and some moldings, MDF is a great choice. Surface faces come from the factory sanded to 150-grit or better, essentially paint-ready. I scuff-sand the surface quickly with 120-grit or 150-grit sandpaper to remove dirt and grime and to provide for better adhesion of the primer coat.

Solvent-based primer for the first coat. Shellac, lacquer, or oil-based primers are a must for the sealer coat on MDF that will be painted. After the surface has been sealed and sanded smooth, you can use a water-based paint.

Solvent-based primers (oil-, alcohol-, or lacquer-based) are a must. Never use a water-based product for the initial finish coat. The wood fibers will swell too much when they absorb the water, and you’ll get what is, in effect, raised grain on the surface that will not sand out. After the surface has been sealed with something else, though, a water-based paint will not affect the MDF adversely. I use latex paint over properly sealed wall paneling and trim molding, but for painted furniture or cabinets, I prefer the finish quality of oil- or lacquer-based paint applied with a spray gun.

Router-bit scraper. Excess compound is easy to scrape away using the same router bit that cut the shaped edge.

The only real difficulty that arises when painting MDF is what to do about the edges, which are more porous than the surface—similar to the end grain of lumber—and drink in most of the finish. I’ve known woodworkers who go to the trouble of edge-banding the MDF. That approach takes more time than the method I prefer, and, no matter how well the edge-banding has been applied and trimmed, a seam still may show at the very edge.

You don’t need a putty knife. A finger is the only tool you need to fill the edges of MDF with the soupy drywall compound. Sand the surface and the sharp ­corners of edges before filling them.

I use drywall compound to fill the edges, whether they are cut squarely or shaped with a router bit, and I apply the compound liberally with a finger or the palm of my hand. Unlike spackle or conventional wood putties, drywall compound has a soupy texture, so it’s a little sloppy going on. But after it dries, it sands off easily.

For edges shaped with a router, you can use the same router bit as a scraper to remove the excess globs of compound before touching up the edges with 220-grit, silicon-carbide sandpaper.

William Duckworth is a woodworker in Woodbury, Conn., and a former contributing editor to Fine Woodworking.

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Published at Thu, 05 Oct 2017 17:11:10 +0000

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