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.
