Number 5 grind is about the coarsest that is usually used in kintsugi. #5 is also the point where you need to switch to a thicker hardening lacquer. You can’t really use regular, high quality lacquer as it is too thin. Using coarser metal than #5 is done sometimes. You will notice if you compare the magnified images of keshifun, #3, and #5 that the grains are getting bigger and the areas between the grains is turning more reddish since they are being filled with lacquer.
The photos show a reddish tint to the gold which is caused by the lacquer.