This is the simplest thing you can do to any diatonic harmonica to get the most improvement.
Use 220 grit sandpaper and a flat surface. No need to buy a precision surface plate made of granite. Just something very flat. And one quick pass with 220 grit is all you need. More passes and a finer grit will actually make things worse and make the surface round, albeit shiny and polished - which is not what we want.
For more improvement, use one of my Dark combs:
https://harp.andrewzajac.ca/Combs