Use a Sharp Saw Blade
You can't cut perfect miters with a dull blade, one with too few teeth or one that's designed for ripping. Check your blade for sharpness by cutting a 45-degree miter on a 1x3 or larger piece of oak or other hardwood. If the blade cuts smoothly with very little pressure and leaves a clean, almost shiny cut with no burn marks, it's sharp enough to cut good miters. When you check your blade or shop for a new one, look for one labeled as a “trim” or “fine crosscutting” blade. A 10-in. blade should have at least 40 teeth, a 12-in. blade at least 60. If the blade is for a sliding miter saw, be sure the teeth have a hook angle of zero to negative five degrees. Teeth with a neutral or negative hook angle are less aggressive and safer for sliding miter saws. Expect to spend at least $50 for a carbide-tipped blade that'll perform well and last.
Tweak the Cut
Even on perfectly square corners, 45-degree angles won't always yield perfect miters. Wall corners can be built up with corner bead and compound, and window and door frames can slightly protrude or be recessed behind surrounding drywall. That's when you have to start fiddling with the angles to get a tight fit. In most cases, you'll be making adjustments as small as a quarter of a degree. If the gap is small (about 1/16 in.), recut one side of the miter. If the gap is larger, you'll have to recut both boards or the trim profiles wont line up.
Glue and Sand for a Seamless Fit
Here's a trick to make miters look great, but it only works if you're installing raw trim that will get finished after installation. Apply a thin layer of wood glue to the end grain of each piece before you assemble them. Use a damp (not wet) cloth to remove excess glue from the joint. Sand over the miter with a small piece of 120-grit sandpaper. Sand across the joint and finish up by carefully sanding out any cross-grain sanding marks by moving the paper with the grain from both directions.The sawdust from sanding will mix with the glue to fill any small gaps. Sanding the miter will also even out any slight level differences and make the job look more professional.
Burnish the Corner
If your baseboard or crown molding has a slight gap in the outside corner miter, you can hide it by rubbing the tip of the miter with the shank of a screwdriver or nail set. The bent fibers will disguise the gap, and the slightly rounded corner will be less likely to get chipped or damaged. The best way to prevent this problem is to cut your outside corner miters about 1 degree sharper than the actual angle so the tips of the miters touch. This will leave a tiny gap at the back of the miter where it's barely noticeable.
Fit One Miter at a Time
Whether you're edge-banding a tabletop as we're showing here, trimming out a window or door, or installing baseboard, it's always best to fit one miter at a time whenever possible. Start with a scrap of molding with a miter cut on it as a test piece. When you have the first miter fitting perfectly, mark the next one. Then cut and fit the adjoining miter before you nail either piece. For edge banding, work your way around the project using the same process for each edge piece.
Guess and Test
There are all kinds of ways to find odd angles, but most carpenters simply make a guess and then cut a pair of test pieces to see how lucky they are. The angle of these two walls looks to be less than 45 degrees. A good guess would be about 30 degrees. Divide 30 by two to arrive at the miter angle, and cut a couple of scraps at 15 degrees. Here there's a gap in front, so we need to increase the angle slightly and recut the scraps at 16 degrees. When you've zeroed in on the correct angle, the scraps will fit perfectly, and you can then cut the actual moldings.
Mark, Don't Measure
Holding trim in place and marking it is always more accurate than measuring, often faster and it eliminates mistakes. This is good advice for other types of carpentry work too, like siding, laying shingles and sometimes even framing.
Use a Shim to Cut a Back Bevel
Cut a back bevel on miter joints that are open in front but touching at the back. To create a back-beveled cut on a standard miter saw, place a pencil under the molding. If you have a compound miter box, tilt the blade a degree or two to cut the back bevel.
Smash Protruding Drywall
Occasionally window and door jambs end up slightly recessed, which causes trouble when it comes time to install trim. Correct minor level differences by either bashing in or cutting out the drywall along the edge of the jamb. But be careful to avoid going beyond what will be covered by the trim. If the level difference is greater than about 3/16 in., nail thin strips of wood, called jamb extensions, to the jamb to bring it flush to the wall surface.
Use a Brad Gun for Best Results
It's hard to beat a nail gun for perfect miters, especially if you're not skilled with a hammer. Trim nail guns allow you to hold the moldings in perfect alignment while you pin them in place. If you can afford only one trim gun, buy one that shoots thin 18-gauge nails up to 2 in. long. Fifteen- and 16-gauge nailers are good where more strength is needed, such as for nailing jambs, but the thicker brads make larger, more conspicuous holes and can crack thin moldings. Use shorter brads to nail the molding to the jamb, and long brads along the outside edges.
Don't Nail Too Close to Ends or Edges
Even with an 18-gauge trim nailer, you can split the molding if you're not careful. Avoid nailing less than 3/4 in. from the end of a trim piece or less than 1/4 in. from the edge.
Pin the Miter Before Nailing the Outside
In a perfect world, you could nail the trim flat to the wall and the miter would look great. But in reality, minor variations in level between the jamb and the wall often interfere. To solve this problem, start by pinning the inside edge of the trim, making sure the miter joint is pressed tight together. Then, while the miter is still tight, drive a pair of brads through the outside corners at opposite angles to pin it.
Shim Behind the Miter
If there's a slight gap between the molding and the wall, don't press the trim tight to the wall and nail it; the miter joint might open up. Instead, slip a thin shim between the molding and the wall. Then nail the outside edge of the trim. If the gap and shim are visible, fill the crack with caulk before painting.
Brad Nailer Helper
Holding trim in place to mark it for length is faster and more accurate than measuring. But that's not easy to do with long pieces of trim. When you're cutting miters and need to hold the end of a long piece of casing in place while you mark the far end, just pin it with your brad nailer. It doesn't take much. If you're putting up 3/8-in.-thick trim, just tack it with a 1-in. brad. After marking, pull the molding loose. You'll have to pull the nail and fill one extra nail hole in the trim.