When making through-mortises by hand, one of the occasional problems is that you get a little mallet happy, you drive the mortise chisel a little too deep and you blow out a piece of grain on the exit side.
Or you drive a too-tight tenon into the through-mortise, the tenon hits the rim of the exit hole and the grain blows out. Or , when making angled through-mortises , your chisel lifts up the face grain when you are bashing out the acute side of the mortise. The results are anything but cute. Here’s how I repair the damage.
The most important step is to make the repair immediately. If you put off the fix, one of two bad things can happen.
1. You can lose the piece of loose grain.
2. The wound will collect dust, or the strings of torn grain will get knocked around. If either of these things happens, you’ll never get the piece back together.
The first step with this repair is to clean the wound. When mortising, you can drive some chips into the gash. Get a knife or a chisel and clean out the junk you might have forced into the split. Sometimes you have to make the split worse to do this. That’s OK.
Get your glue. I use yellow glue for most of these repairs. It’s fast-drying and strong. Cyanoacrylate is faster, but I have found that it’s also quite brittle. One good knock and the split will open again.
Pry open the wound a bit and wick the glue into the split. If possible, I’ll hold the workpiece so that gravity will help wick the glue in. If you can’t get the glue to wick into the split, pry the opening a little more. Or thin a little yellow glue with water. (This will also slow its drying time.)
Press the grain flat with your fingers and ensure that glue is squeezing out where it should , all around the split. Then tape the grain down. I use blue painter’s tape. Then put a clamp on the repair. Or use the pad of a holdfast if it’s a chair seat or some other large piece of work where your clamps won’t reach.
You can remove the clamp after an hour or so. I don’t recommend you work the split until the glue has reached its full strength, which usually takes a day.
– Christopher Schwarz