Sloppy slots can be a headache — here are three ways to mend them.
Man looking at gap in joint

Offer One Solution

Use veneer or rip a shim of matching stock to fill the remaining gap, sanding, if necessary, to create a tight fit. Place the shim on the side where it will least likely be seen. For example, few people will ever look under a low shelf, so put the shim below it. Install a shim on the top side of a high shelf. Glue the shim to the shelf and dado wall, trim or sand it smooth, and you'll likely be the only one to ever know it's there.

End of Cabinet
Cut or sand a shim to fit snugly; then score it flush with a utility knife. Remove the shim and trim it to size on your workbench.

Wedges Work Too

For gaps smaller than 116 ", use a wedged spline to spread the ends of the inserted workpiece, tightening the fit. Begin by routing a 18 "-wide, 12 "- to 34 "-deep slot centered along the board's end using a slotting cutter. (You can also cut a 18 " kerf using your tablesaw, but long, unwieldly workpieces will need the support of a tall auxiliary fence.)

Next, cut a wedged spline with 5° bevels along each face, with the narrow edge the same thickness as your slot. Gently tap the spline into the slot. This might require some trial-and-error trimming or sanding to achieve the perfect fit.

Gap in side of board
Ding in side of board
Tapping the wedged spline into the slot spreads the workpiece end to tighten up any loose fit.

Time To Start Over?

If the gap is too wide for either of the previous fixes, and you don't want to scrap the part, fill the dado with a strip of similar wood that closely matches in color and grain pattern, as shown below. Next, recut your dado to the correct width, making test cuts in scrap to confirm the fit. If possible, position the new dado so any sliver of the filler strip that will show is on the side that will be seen the least.

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Cut a filler strip from scrap stock thatclosely matches in color and grain.Glue it in place; then plane or sand it flush.