Kokuso & Sabi: Filling, Reinforcement, and Repair Paste
Two putties do almost all of the structural filling work in urushi craft, and they are not interchangeable — kokuso is coarse and structural, sabi is fine and cosmetic/final-fill. Both also happen to be the working vocabulary of kintsugi ceramic repair, since kintsugi is essentially urushi substrate technique applied to broken pottery instead of wood. (JAANUS, kokuso-urushi)
Kokuso — for gaps, chips, and joint reinforcement
| Component | Role |
|---|---|
| Mugi-urushi (flour + water + raw urushi) | Binding base |
| Wood powder (fine sawdust) | Bulk aggregate, any fine wood works |
| Polishing powder (fine ground rock) | Structural filler |
- Mix the three components; the source recipe does not give a fixed ratio — work to a stiff, moldable paste.
- Apply in thin passes of only 0.5–1 mm at a time with a bamboo spatula, wiping the spatula frequently on tissue as material sticks to it.
- Let each pass dry at least overnight before adding the next. This is not optional: a kokuso layer applied too thick traps air and the interior never fully cures.
- Build up until the fill sits level with — or very slightly below — the surrounding surface.
- Only after the fill is complete, sand flat with 320-grit paper. Don't sand between kokuso layers.
- Leftover kokuso keeps about a week rolled into a ball, wrapped in plastic film, stored cool and dark.
(Full process: Tsugu Tsugu, Filling the Deep Chips with Kokuso)
Sabi — for small holes and final surface leveling
- Put fine polishing powder (traditionally whetstone-cutting waste) on a mixing palette.
- Add water a few drops at a time with a dropper, kneading with a spatula until it reaches the consistency of hard mayonnaise.
- Add raw urushi equal to roughly 60% of that paste's volume, and knead quickly.
- Work fast — sabi-urushi begins hardening as soon as it's exposed to air. Keep it covered with a spatula "lid" between uses.
- Apply with a bamboo or specially-shaped spatula. On glass, unglazed ceramic, or other surfaces you don't want stained, mask everything except the repair area with tape first.
- If applying with a brush rather than a spatula, work quickly and wash the brush tip in oil immediately after — sabi hardens onto brush hair fast.
(Full process: Tsugu Tsugu, Filling Small Holes with Sabi-Urushi; see also the general overview at kintsugiblog and megu.space.)
