On occasion one comes across a split or crack that for one reason or another cannot be closed with pressure from clamping. It is important to determine the cause of the split and fix that problem first. In many cases fixing the cause of the split will repair the split however on those occasions when it does not this is one way among others that may be appropriate. It works best when the split is wedge shaped as in this photo. This is the only way to fill a split that will take stain and finish exactly the same as the adjacent wood.
Step one is to clean any dirt, debris or old glue out of the split. This can be done by dragging the back edge of a tapered knife blade through the split.
Step 2 is to make a tapered filler strip out of the same wood you are filling.
To make the strips I plane the appropriate angle on the edge of a board of the correct lumber species. I adjust the lateral lever on a hand plane to match the taper of the split. I then cut off the strip with the band saw.
The strip is further fit with a card scraper. It is important that the sides of the strip contact the sides of the split. You do not want to drive the wedge into the split, it should fit snugly without driving the split open further.
The strips are glued in place with hide glue, hide glue will not negatively affect the wood’s ability to take stain and finish like other glues.
Waste is trimmed away with a chisel.
Finally cleaned up with a card scraper.
It’s important that wood movement issues have been dealt with or the glued in strips may actually make the problems worse.