For the best appearance, moldings should run in one continuous strip from one corner of the room to the next. But when the room is longer than your molding, a joint becomes unavoidable. Your task, though, is to make that seam as invisible as possible. The solution is a scarf joint. Made with 45-degree cuts, this seam won't take any attention away from the beautiful room it sits in. Learn how to make one for your own home makeover using our helpful tips below.
Below, we give you the steps you'll follow to make a scarf joint, but there are several other things you can do to disguise the joint. If you're joining wood that will get a clear finish, join pieces that have the same subdued grain pattern. Joining a wildly grained strip with one that has straight grain will make the difference immediately apparent.
If possible, locate the joint where a bed, bookcase, or other large piece of furniture will block it. That way, the only time you'll see the seam is on moving day. Behind the door is another good hiding spot, but be careful of positioning a joint too close to a corner. A seam closer to a corner than 16 inches may look as if you're fixing a mistake — not making a planned extension.
Painted scarf joints are easy to conceal, but they still require careful workmanship and sanding. Paint is merely a finishing coat, not a cure for a poor fit.
If you apply stain and a clear finish before cutting the scarf, you won't have to sand the joint smooth. Although this will save the effort of applying the finish after installation, it means that you'll need to spend extra time to fit the joint as closely as possible. Touch up the joint's ends with a stain pen to eliminate the appearance of raw wood.
Select the location for the scarf joint, positioning it over a stud, if possible. Mark the stud's edges and centerline onto a piece of tape applied to the wall. It's also helpful to find a place that you plan to have covered by a couch or a dresser.
Fit the end of one molding piece into a corner, then mark the seam cutting line. In this example, the corner at right is fitted first. To help your cutting, mark diagonally across your molding piece.
Make the first half of the scarf joint with a 45-degree miter cut that opens away from the wall. Nail this piece to the wall at the other stud locations along the wall.
Fit the end of the second molding strip into its corner, and mark the location of its scarf joint. Notice that the miter runs in the opposite direction as the cut in the first piece you installed. Make several cuts in order to sneak up to a perfect fit.
Drill pilot holes for the nails that will secure the seam and hold the molding to the wall. Don't neglect this step because the glue makes the joint slippery, and the pieces can shift out of position as you're driving the fasteners. Slightly angle the pilot holes so the joint doesn't slide apart when you drive the nails.
Apply glue to both ends of the joint. Get both nails started through the pilots before you drive either one of them completely. Countersink the nails as little as possible. Sand the joint smooth and touch up the paint.
Load abrasive paper into a hard rubber sanding block to level the flat surfaces of scarf-joined moldings. Don't concentrate sanding pressure right at the joint line—feather out the sanding several inches to each side of the seam line. Remove dust with a tack cloth, then apply primer to the raw wood.
Smooth curved surfaces with sandpaper wrapped around a rod or cylinder that matches the molding. Some good cylinders include round pencils, dowel rods, and PVC plumbing pipe. Pressure-sensitive abrasive (PSA) sticks to the cylinder, making your work easy. Spray adhesive on ordinary sandpaper is a good alternative to PSA.
On some large pieces of trimwork, such as crown molding, you can join lengths prior to installation. Doing this significantly reduces the amount of work you need to perform while standing on a ladder. As a result, you get better joints with less work.
Accurate cutting of the opposing miters for the joint is still an absolute necessity. Follow the recommended setting time so the assembled joint will develop strength in the glue joint before proceeding with installation.
For the wood gusset, cut the scarf joint, apply glue to the cut ends, and press them together firmly. Align the assembly against a straightedge and apply strips of masking tape to keep the joint shut. Glue and screw a plywood gusset over the seam. As the size of the molding increases, choose thicker plywood, but make certain it doesn't interfere with installation clearances. Steel mending plates are a good alternative to plywood, lending excellent strength with minimum bulk. Choose coarse-threaded screws for maximum holding power in softwoods. During installation, don't attempt to drive or shoot nails through the metal plates.