A spline repair can clean up an imperfect joint and make it stronger.

Let’s be honest. Miter joints look easy enough, but in reality, getting them just right can be a pain. Some days, you can use all of your experience and expertise and take all the necessary steps to create a perfect miter joint and still end up with a result that’s far from perfect. You glue it together, wait for it to dry and finally take the clamps off, sure that you’ve done a great job, but instead you find that it didn’t quite come together as well as it did during dry fitting. 
 
It’s happened to me more times than I would like to admit. When it does happen in the Dark Horse shop, we have a strategy. We cut a dado through the joint at a 45-degree angle; the width of a saw blade is just about perfect. The cut doesn’t need to be deep; somewhere in the neighborhood of 1/4 of an inch to 3/8 of an inch is fine. 
 
We then make a spline out of the same material and glue it into the dado, making the spline a little long and wide as well. A bit of blue tape works great as a clamp for this. When the glue has dried, use your favorite method to remove the excess; at Dark Horse, we use a combination of hand planes and sanders to get the job done. 
 
Taking these steps will lead to a perfect miter, and you’ll have the added bonus of ending up with a stronger joint, thanks to the full-length spline. This method works well for both paint-grade and stain-grade projects.