To fill a small chip, pin hole or gap in a break, you use Sabi. Sabi is much finer than Kokuso, but  this technique is not strong enough for a large repair.
 
You need, 
​Tonoko (polishing powder), 
Ki-Urushi, 
A dash of water,
Acetone
 
Tools
Disposable gloves , 
A white plate, or glass work surface, 
A spatula , 
＃3000 wet and dry sandpaper, 
Box to keep your plate safe 

Preparation
When you use urushi, always wear disposable gloves.
 
Apply diluted Ki-urushi with gum turps (50:50), using cotton swab or brush on areas where you need to apply Sabi. Then place in Box. (Keep for a day in a box if you have time.) The photo is for preparation of Urushi Glue, but the work is the same.

Making Sabi / Fine Urushi putty
Make wet Tonoko by mixing Tonoko and water to form a moistened solid ball then add Ki-Urushi little by little to wet Tonoko. Proportion: Ki-urushi : wet Tonoko≒1: 1.2

Fill the chip and gap also cover rough surfaces with Sabi by using a spatula or a stick.
You can make runny Sabi mixing Kiurushi and Tonoko in equal  amount to apply to any tiny gap in the repair. 
Keep in a box for about three days till it sets.  Before it sets completely , clean excess Sabi with acetone.
 

Polish Sabi with sanding paper with water. Take care not to damage surface of item with sand paper. 

Repeat this Sabi application until you achieve smooth surface. 
When you finish application of Sabi, you can move on to Undercoat / Middle coat (Shita-nuri, Naka nuri)

If it is small chip or gap of break, you can fill them with  sabi directly , no need to fill with kokuso before sabi.