Make a mixture to match your finish
To fix gaps, cracks, and small holes, make your own filler by mixing sanding dust with hide glue, linseed oil, or a bit of your final finish. Here, we used polyurethane varnish and powdery dust that we reclaimed from the bag of our belt sander.
Make your filler into a thick paste, and force it into the gap with a putty knife. Sand it to aid in drying and to level the surface. Use the same grit that you used for your final sanding step.