Q. My company installs prefinished millwork in a commercial environment, so we rely heavily on MDF-veneered products. Is there a fastener we can use in our pneumatic nailers that will prevent — or at least minimize — the "mushroom" that occurs when the fastener is driven into the MDF?
A. Charles Stout, director of Laboratory and Certification Services at the Composite Panel Association, responds: When a fastener enters wood, the fibers in the fastener's path are displaced into air spaces that surround each fiber. But MDF is a compressed cellulose mixture; when a fastener enters it, the material in the fastener's path has nowhere to go but up and around the fastener and out — the so-called "mushroom." The degree of mushrooming that you get with MDF varies depending on the particular type of MDF being used, and the size, type, and design of your fasteners.
MDF actually comes in a range of densities, which has an impact on how likely it is that individual boards will mushroom when fasteners are driven into them. For example, standard-grade industrial board doesn't have a uniform density, but typically is manufactured with a softer core and harder outside surfaces, and is more prone to mushrooming. On the other hand, because of the way the fibers in molding-grade MDF have been processed, the resulting substrate has uniform density from surface to core. Puckers and mushrooms are less likely to occur in molding-grade MDF because the less-dense material at the site of the nail head allows for more material expansion.
Slim, "needle-style" 18-gauge brad nails work best in pneumatic nailers. Staples are also an option; use fine-gauge, narrow, coated crown staples with chisel points. To minimize puckering, your nailer should be set to drive the fastener as flush to the surface as possible. And for the most consistent results, hold the nailer vertical to the surface being nailed.
You can, of course, use screws with MDF. Be sure to predrill pilot holes that are 85 percent to 90 percent the root-diameter of the screw and at least equal in depth to the length of the screw, countersink as necessary, and don't overtighten the screws. Instead of using standard wood screws, use special MDF screws, type A or AB sheet-metal screws, Twin Fast screws, or fully threaded screws designed for use in particleboard.
Finally, some finish carpenters recommend using a sharp chisel to trim off any mushrooms that occur when nailing MDF. This reduces scarring of the surface and the amount of sanding or scraping required to prepare MDF for a paint finish.
