Hi! Thanks for visiting the site. I attached this broken handle of a mug a while ago, usung “mugi urushi”. Today, I decided to polish a bit and paint the first layer of Urushi.

As you may not be able to see on this photo, the line is extremely thin, less than 1mm.
Here are two items I use today, a black block of charcol “sumi” suruga, imported from Japan, and a polishing stick No. 700 “Asakichi Toishi” from “Tsutsumi Asakichi Urushi” shop in Kyoto.

The top part of this stick is 1mm x 4mm. You can see how narrow the line is.
I use “sumi” charcol as well to polish fragile surface as this one. The advantage is that it is natural material, so simply I feel better, while as it is natural, the quality diverses on each charcol block. Also, it costs quite a bit. Some kintsugists say that you only need a little bit of “sumi” such as 10g, or 50g, but I disagree. If I try to make the surface as the way I want, I need quite a lot of them. It costs me about 100 euros for 300g.
In conclusion,
Sand papers are easy to find in any DIY shops. I use them for polishing on certain areas, however I feel that is not enogh to make the surface smooth enough. I feel that the tools specific to polishing Urushi can bring you better results.
Asakichi sticks are quite useful to polish very thin lines and small areas without damaging surrounding surface.
If you prefer more natural and traditional items, you can try various kinds of “sumi” charcols, not only Suruga, also other types depending on the polishing results you aim. There is also a kind of grass/plant called “Tokusa” to polish even more fragile surfaces. I personaly have never used them. You can shop from one of the Urushi shop also in Kyoto.
I hope you find this information useful!
Happy Kintsuging 🙂