Hints on Trimming With MDF

Trim can dress up any building project.

Medium Density Fiberboard (MDF) is an engineered material made from wood fibers aggregated in a mix of urea-formaldehyde resins. The small fibers have randomized sizes, orientation and densities, which make MDF sheets resistant to warping, splitting and grain-related defects.  MDF is less expensive than similarly-dimensioned plywood sheets and dramatically less expensive than solid wood trim materials. In addition, MDF trim stock is typically primed on at least three faces.

Preventing Off-gassing

One reason MDF trim products have been somewhat slow to be embraced by consumers is that the resins used to bond it are based on formaldehyde, a known carcinogen. MDF could conceivably become the source of indoor air quality problems if it is not properly sealed before application. To ensure that MDF trim will not exhale formaldehyde, the best practice is to paint all sides of the trim with a high-quality acrylic or alkyd paint. A thorough coating with either of these products permanently seals MDF trim.

Fastening MDF

In many ways, MDF is more forgiving than wood. It has no grain or knots and is not as prone to splitting as natural wood, if nailed near an edge. However, it is not as resistant to damage from impacts and scrapes as natural wood.  If a hammer blow glances off a nail head, it will almost certainly leave a visible depression  in the trim profile.  When hand nailing, leave all nail heads at least 1/8-inch proud of the trim surface. Come back through and set nails with a tap set.

Trimming Curves

A relatively new MDF product, BendyMDF, comes with pre-sawn grooves in the back profile, making it fast and convenient to use when trimming out curved features. Because uniform grooves are manufactured into the trim product, avoiding a kinked or segmented appearance is much less difficult.  However, the additional surface area on the back of the MDF creates more pathways for the escape of volatile organic compounds, making proper sealing especially important.

Fragility of MDF

MDF has good compressive strength, which is one reason it is favored as a substrate for laminate counter tops.  However, it is highly vulnerable to edge damage. In moving MDF sheets and pre-shaped trim, never rest its full weight on an edge. Be extremely careful about nailing into an edge, and always drill pilot holes before screwing into MDF. When using compressed air powered tools, experiment with the sensitivity of the pressure trigger and nailing force. Excessive air pressure can send fasteners all the way through manufactured MDF trim pieces.

Sawing MDF

Sawing MDF, either with hand tools or power saws, creates an almost unbelievable amount of dust.  MDF particulates are extremely fine, and difficult to clean up, either by sweeping or vacuuming.  Breathing the dust is hazardous. Always wear a HEPA-approved mask, make cuts away from inhabited parts of a home or office and if possible, use a saw with an effective dust management attachment.

References

• Woodweb: Knowledge Base: Working with MDF Trim