When it comes to trim carpentry, the key to making it look good is all in the joinery. You can’t fix or hide a bad joint with caulk, and it’ll always be visible. A good joint, on the other hand, doesn’t need caulk or filler. A good joint is where both faces meet all the way across the trim, without any gaps. Here is how it’s done.
If the walls are gypsum board, there’s mud built up in and on the corners, so you won’t be able to make a 45° miter in each piece of trim work. On outside corners, you’ll wind up with a gap between the faces. I usually set the saw at 45.5°-46°, depending on the amount of mud built up. Dry fit the joint, and then when you have it right, apply some glue then shoot a couple of brads into the joint. The glue is important because wood continues to move even after it’s been nailed to the wall. The glue will keep the joint together in spite of any movement. This is all fairly obvious stuff so far, and applies to most any profile that nails flat to the wall. Inside corner joints are a bit different.
When it comes to making inside corner joints, you’ll need another saw in addition to the miter saw. It’s a hand tool called a coping saw, and is used to cut the profile of the trim so that it mates the other piece of trim like a puzzle. Here is a picture of one that I use. This is the minimum size. It has a six inch blade, and a five inch throat. You’ll also need a fairly aggressive blade, 15-20 tpi. You can set the blade to cut on the push stroke or pull stroke, whichever you’re comfortable with.
Now set the miter saw at 45º and make your cut on the right end of the trim. You’re going to use the coping saw to cut the line where the miter cut meets the face of the trim. So set the piece of trim on a work surface no more than about waist high, because you’ll need to be able to hold the trim steady with your left hand. Let the cut end of the trim hang out beyond the table by about six or eight inches. Hold the trim down securely with your left hand, and with the coping saw in your right hand, start cutting at the top of the trim, holding the coping saw at about a 15º angle, so the handle of the saw will be slightly pushed away from you. You don’t have to push hard or move fast through the trim, as the saw will do the work for you. You may need to practice several times to try to get the feel for it. There is some finesse involved, so don’t get frustrated if it winds up taking you most of the day practicing. The result will be well worth it. Put the coped end of the piece you’ve been working on at 90º to another piece of the same profile. It should be a perfect fit. Making your joints this way is the preferred method for any other trim carpenter I know.
This technique should take care of most any profile that goes flat on the wall. Next time, I’ll write about what it takes to cut, cope, and hang crown moulding.