I cut a few dovetails making my bookcases. 70 boxes over the course of a year and change. Practice helps, but as Chris Schwarz has said, everyone has a certain number of crappy dovetails in them, and the only way through is to cut them and get them out of your system. Eventually, they’ll show up less often.
Most of these cases, I didn’t bother to fill any gaps, as they’re close enough to each other that people won’t see the gaps without a dentists mirror to look between the cases, and I set the best ones on the ends where they’re visible.
But if I have to fill gaps, I typically collect the sawdust from cutting the tails (so it matches) and jam that into the gaps and then add CA glue to bond it. Maybe a shot of accelerator if I’m in a hurry. Works pretty well, even when working small. This is ash with dovetails that were pretty gappy. Just the width of the kerf looked like a huge gap when I cut the wrong side of the line on one of them. That hinge is a 12mmx6mm (½ x ¼ inch) hinge, so the tails were close enough together that a ¼ inch chisel was too big to clean them up. The stock was roughly 3/16” thickness, so I was cutting dovetails with a 24 tpi gent saw. I used a 2mm diamond file to clean things up, then filled the gaps with sawdust and CA. The recipient was thrilled.
Note that I always cut out the waste with a coping saw, rather than paring with a chisel, clean up with a rasp or file, then glue them together with everything proud on the corners. Pins and tails are sticking out all over. Then I take the box to the belt sander and “trim” the protruding pins and tails with that (there’s usually a belt of 60 grit on there, though if it’s an old one, it behaves more like 200 grit). Plenty of fine dust available for filling.
