There are several steps in kintsugi where you need to either sand down or polish parts of the piece.
The initial use of sanding is after you have either stuck the pieces back together or done any kind of work wherein you have to smooth the seam. You can use either sand paper or specialized whetstone type materials. Sand paper is cheaper for the initial stages but as you fine tune your seams it is easier and more precise to use soft whetstones.
Soft whetstones come in the following meshes, #400 #600 #800 #1000 #1500 #2000 and #3000

There is almost no difference between the different meshes as far as appearance goes so I will just put this one photo up. If you have them you have to write on them the mesh as you won’t be able to tell them apart.
I use the meshes #400-#600-#800 to do only the roughest work on the seams. I never use these meshes on graded metals as they are too rough and will destroy the metal. If you use any of the soft whetstones on unglazed ceramic or directly on rough sabi you will wear them right out and have very little effect. You should use the rougher meshes to smooth out sabi but don’t expect them to last if the surface is very hard and rough.
I use the meshes #1000-#1500 to do the initial polishing of graded metals. Use #1000 delicately since it can rip the metal off the lacquer. The #1500 will shine graded metal to your final state. After that you need to use #2000-#3000 to polish away the scratches from the #1500 and get to almost your final shine.
Note that polishing metal is only for graded metal, not for fine powder. If you polish fine metal it will come off and you will ruin it.