The keys to making a sound MDF joint
Question provided by Robert Persing, Skillman, N.J.
A: Yellow glue and screws make solid joints in MDF, Bob, but not just any screws. Bear in mind that this material has less screw-holding power than solid wood or plywood, and tends to split.
When joining 3/4" MDF, we recommend using #8 deck screws (featuring straight shanks and deep threads) at least 1-1/2" long. Locate the screws no less than 2" from the ends of the pieces to avoid splitting. Bore a 5/32" shank hole through the top piece of the joint for each screw, and countersink both sides. One countersink seats the head flush with the exposed surface and the other makes room for any material pulled up from the bottom piece of MDF when you drive the screw. Next, spread glue on the edge of the bottom piece, and clamp the joint together. Finally, drill 7/64" pilot holes into the bottom piece, centered in the shank holes and as deep as the screw length. Drive the screws, and wipe away any squeeze-out with a damp cloth.