Sorry about the poor pictures I can never take good blade pictures
@ Stugumby
As some of the Guys have said use fine 'sand paper'
Back it with a wood block and piece of leather because of the concave grind
Here's a leatherman with very scratched blade tip (that's not dirt it's lots of very fine scratches)
I started with 120grit, block of wood and some hardish leather (you might need finer)
The leather was too hard so I switched to a softer piece
So I worked through the grits 120 250, and 600 then polished it on a buffing wheel
So that's easy I have finished
No, it looks OK but I could still see scratches
So go back a step and to my 250 grit glued to a board
As you can see it did the hard work but leaves a hollow
A piece of 600grit on a soft leather pad on top of my bench
That's looking better
After a trip to the buffer
Just need to do the other side then sharpen
Hope this helps