10 tips for using MDF wood

Medium-density fibreboard is the most versatile building material I know of. Because it’s inexpensive and fairly durable, medium density fibreboard is a good choice for practical projects like shelving and storage cabinets. But MDF wood is great for decorative projects too. The smooth surface of medium density fibreboard is perfect for painting, and a router leaves crisp profiles with no splintering, burning or tear-out.

Over the past 15 years, I’ve used MDF to build everything from crude shelving in my shop to fancy trim in upscale homes. I’ve even used it for furniture and ornate millwork like detailed trim boards. In fact, my own home is entirely trimmed out with MDF mouldings made from about 50 sheets of MDF. Yes, I’m a fan of the stuff. This article will cover the most important things I’ve learned about working with MDF – and help you avoid some of the frustrating mistakes I’ve made.

MDF is basically sawdust and glue, fused together under pressure and heat. It varies in colour from tan to chocolate brown. Common thicknesses range from 3mm to 18mm, but can go up to 25mm or 32mm thick. Full sheets are 3600 x 1200mm, while other sizes include 2440 x 1220mm or 1200 x 900mm. Working with MDF is no different from working with wood or plywood; you use the same tools to cut and shape it.

Check out these 28 brilliant woodworking tips for beginners.

When a job calls for painted trim, I almost always cut costs with MDF. Even inexpensive wood can cost four times as much. To make trim, I cut MDF sheets into strips and shape the edges with a router or router table. With the right bit, I can create just about any trim profile, simple or fancy. Hardware stores also carry ready-made MDF wood trim for less than the price of solid wood.

Find out how to install skirting.

Here’s one of my favourite tricks for painted trim, cabinets or even furniture: Use MDF for the large, plain parts and dress them up with wood mouldings like base cap, coves or base shoe. That gives you the money savings of MDF without the time-consuming work of making MDF trim from scratch.

The wainscoting shown here, for example, is simply panels and strips of 18mm MDF trimmed with inexpensive, small-profile pine mouldings. The cap rail is likely to take a beating from chairs, so I make that from wood instead of MDF. Once coated with primer and paint, the wood and MDF wood parts will look exactly the same.

There’s one thing you’ll really hate about MDF: the fine, powdery dust that invades your clothes, hangs in the air for hours and clings to every surface like a coat of frost. Cutting MDF is a dusty job, but routing it is even worse.

Whenever possible, I cut and rout MDF outside. When that’s not possible, I drape sheets of plastic over shelving and other hard-to-clean areas in my shop and use a fan to blow dust outside. When installing trim in a room, cover doorways, close air vents and expect to vacuum every surface when you’re done, even the walls. Clean your vacuum filter often—the fine dust plugs filters quickly. And a tight-fitting dust mask is essential.

Here are 8 alternative uses for sawdust.

MDF is kind of like a cream-filled biscuit: two hard faces with a softer core between them. That soft core splits easily when you drive a screw into the edge. The hard face presents different problems for screws. If you don’t drill a countersink recess, the screw head may snap off before it sinks into the MDF. Or, if the head does sink, it might push up chips. The cure for both problems is to use a countersinking drill bit.

Here’s everything you need to know about drilling techniques.

I blame my hernia on MDF. A full sheet of 18mm MDF weighs about 56kg, and I’ve lugged lots of them from my pickup to my shop. But there are ways to avoid hernia surgery:

Buy half or quarter sheets instead of full sheets.

Some timberyards and hardware stores sell MDF shelving. I like to slice these long, easy-to-handle shelves into trim stock.

Some hardware stores and timberyards will cut full sheets into manageable sections at no extra charge.

The face of MDF is smooth, but the edges are fuzzy like the skin of a peach. If you just slap paint on the fuzz, it will look and feel like sandpaper. So you have to get rid of the peach fuzz before you paint. I have two recipes for smooth edges: one for “good-enough” edges and the other for edges that will get a high-gloss finish.

Here’s the good-enough process I apply to most projects, including trim: First, lightly sand the edges with 100-grit paper. Foam-backed sanding pads work great on routed profiles. Then prime the MDF. Use a solvent-based primer only. Water-based primer can raise small blisters. the primer dries, sand off the fuzz with 100-grit pads.

A couple of light passes is all it takes. You can usually sand after a couple of hours, but let the primer dry overnight for smoother results. After sanding, wipe away the powdery dust with a damp cloth and you’re ready to paint.

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The face of MDF is harder than most woods, but the inner layers are soft. So edges, and especially corners, are easy to crush. That means you have to handle it with more care than timber or plywood. Also, avoid scratching the face. Light scratches stand out like a sore thumb on the ultra-smooth surface, so you have to sand them out completely before priming.

And wear gloves when handling MDF wood, especially when carrying heavy sheets. MDF edges can be sharp enough to cut skin – I’ve got the scars to prove it.

MDF doesn’t stand up to moisture well. A few water drops will raise small bumps on the surface. A long soaking will make it swell to twice its original thickness. So MDF is a risky choice for skirting boards in entryways and trim near tubs or sinks. My all-time greatest MDF mistake was using it for windowsills in my own home.

If you use MDF as skirting board, be sure to paint the lower edge before installation. That will provide short-term protection against occasional spills. Also install the skirting board about 6mm above the floor and then cover the gap with wood base shoe moulding. There are moisture-resistant versions of MDF, but they’re hard to find. To find manufacturers and dealers, search online for “moisture resistant MDF.”

MDF is commonly used for shelving in wardrobes and cabinets because it’s inexpensive and smooth. But MDF isn’t as stiff as plywood and will sag over time. So when I use MDF shelving for heavy loads, I simply beef it up with wood. First, I rip a 1×4 right down the middle to get two strips about 40mm wide.

I glue one strip flat against the underside of the shelf to stiffen the back edge. Then I glue the other strip (nosing) to the front edge. The MDF edge will absorb a lot of glue, so spread on a light coat, give it a minute to soak in, and apply another bead before you add the wood nosing.