Most of Japanese chefs use Comet cleanser powder type or liquid type to remove stain and rust.
It does not work perfectly with just one time, but if we do it regularly, it works very well.
Putting on the blade on the cutting board, and make sure it is stable, and the blade edge is safe position, and pour Comet cleanser with sprinkle of water if you need it, and scrub the blade with thick soft dish washing sponge or thick towel.
Thick means preventing cut our fingers.
After scrub the one side of blade a couple minutes or some more, turning over and set the other side of knife blade on the cutting board, and make sure the blade is stable, and scrub the blade a couple minutes.
And then we can scrub the handle too, if we want it.
If the rust is worse, we can use rougher side of dish wash sponge too (blade might get scratches).
We should always be careful with the blade edge. The blade edge is always on the cutting board.