- Q.Because it's a lot cheaper
than most primed stock, I'd like to use MDF for running
trim. But that means I'll have a lot more butt joints,
especially if I use 8-foot sheets, which I find to be
more manageable on site than 10- or 12-foot sheets,
which we can also get. What's a good way to make
invisible butt joints in MDF when, for example, I'm
installing baseboard or a crown frieze?
A.Gary Katz, moderator of
JLC Online's finish-carpentry forum and a frequent
contributor to the magazine, responds: If
you're working with flat stock, I'd suggest
pocket-screwing all the joints together before
cutting or installing the pieces. I frequently use
pocket screws and butt joints for wainscoting rails
and built-up baseboard, as I've found that this is
the best way to ensure tight-fitting, durable
joints, especially in long lengths.
I use pocket screws in MDF casing, too, if the
profile is a simple quirk-and-bead or ogee on only
one edge of the molding and there's enough room to
pocket-screw the casing together.
If the material is milled with an elaborate
profile, there isn't always room for pocket screws,
in which case I cut a 45-degree miter joint and
reinforce the joints with biscuits and glue. Of
course, you also have to use glue in every joint,
and you should be sure to clamp the pieces firmly
before driving in the screws; otherwise, the miters
will slip and the joint won't be flush.