How do I make smooth-bottomed dadoes?

How can I cut dadoes with smooth, even bottoms using my tablesaw dado set? My set leaves ridges even though I just had it sharpened.
—Walter McCoy, Salina, Kan.

Unfortunately, Walter, very few stacked dado sets leave a perfectly smooth, flat channel, even when brand new. In most cases, it's not a big deal because you'll never see the joint after you've assembled the project. But when you need it dead flat, use a dado cleanout bit, shown above, in your handheld router. These bits have a shallow cutter, ¹⁄₈ " to ¹⁄₄ " in thickness, leaving room on the dado's shoulder for the bearing to register against. If your dado needs to be a specific depth upon completion, be sure to deduct the to-be-routed depth when setting the height of your stacked dado set.