Smooth Rough Joints with a Wedge
Here's a trick I've used over the years to improve the appearance of less-than-perfect edge-to-edge joints. The key is a "feather wedge," a scrap of the same species as the workpiece, beveled about 3° on both sides. After applying glue to the slot, gently tap the wedge into place and wait until the glue dries before sanding flush.
—Chuck Hedlund, WOOD® magazine staff