Sandpaper is also popular for sharpening:
it is cheap
it is available in all grits
a flat base is also quickly found: Glass plates are usually very flat
there is a suitable paper for each type of lubrication.
The problem may be clamping the sandpaper without it slipping.
With a belt sander, you can even grind by machine.
Caution:
Do not grind against the bevel!
Risk of overheating with machines.