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Miter Joint Separate‏

Question: I am installing new baseboard and I have a problem with the external corners. They want to separate. How do I prevent the corners from coming apart?

Answer: The best way to hold an external miter joint together is to use a combination of finishing nails and carpenters glue.

Drill pilot holes across the joint through both pieces of baseboard. The diameter of the holes should be just slightly less than the diameter of the finishing nails you are using. The length of the nails should allow the tip end of the nail to penetrate the second piece of baseboard by 1/2 inch. Drill the holes in opposite directions, so that each baseboard (side of the mitered joint) has two nails in it. Apply carpenters glue to the joint. Hammer in the nails so that the heads are just below the surface. The joint will actually be held together by the glue, the nails only serve to hold the joint in place until the glue has set.

Do not nail the baseboard to the wall until the glue has set. Most walls are not true 90 degree angles and if you attempt to force the 45 degree mitered joint that is not strongly glued together into an angle that is not 90 degrees you will open the mitered joint.