It's frustrating when you cut a mortise only to find out your tenon doesn't fit snugly. Luckily, though, there's an easy fix that doesn't require cutting new mortises. The key is to build up the tenon instead.
A simple fix for a round tenon is to glue on a shaving from a hand plane. Then sand the tenon to fit snugly in the mortise.
For square tenons, glue a thin piece of wood veneer to each cheek. Gluing veneer to both checks ensures the tenon will be centered in the mortise. Orient the grain in the same direction to get a strong glue joint, too. Again, once the glue dries, trim the tenon to fit snugly in the mortise.
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