Tokusa(木賊・砥草) or scouring rush is necessary for kintsugi work. To curve lacquer clay or to file off the overrun of lacquer, using sandpaper sometimes would damage the surface of ceramics. But if you use tokusa instead, you don't have to worry about that.
Ms Ma gave me tokusa in her front yard, that I boiled with a bit of salt and dried.
When the lost part of ceramics is too big to repair it trusting one's hunch, I could use 'molding method.'
I got this broken cup at a dump yard of a ceramic factory to practice the 'molding method'.
As one of four edges was gone, I made a mold of a not-broken edge with paper clay. And then I applied the mold against the lost part and filled it with 'kokuso' or lacquer clay.
After kokuso has dried, I shaped it. I didn't finish it with gold powder, because this is just for practice.
This is Ms W's antique. An edge was largely chipped. I put lacquer clay 'kokuso' on the chip and let it dry for two weeks. Then I shaped the kokuso with a carving knife and 'tokusa (Equisetum hyemale)'. It was rather difficult since I had to carve it trusting my hunch following the fringe shape of the bowl.