Use sandpaper on a flat but somewhat soft surface. The idea is that the backing surface has some "give", so when you press the edge into the backing, it conforms to the convex edge and doesn't flatten out the convex bevel. Sort of a self-conforming abrasive. (the pro's use a slack belt... a grinding belt with plenty of slack in it)
Lots of ways to do this... I did this:
1. got a nice quality flat 2"x4" and cut off a hunk about 14" long.
2. found two old rubberized foam backed computer mouse pads. Cut chunks of mouse pad to size, and used contact cement to glue them to the 2x4. Glued down "rubber side up" so it grips sandpaper.
3. cut some small squares of mouse pad to use as grippy "feet" on other side of slab of wood...glued down of course.
I need to cut a thin slice out of front to use to "mount" the paper so it isn't free floating.
Backing of thick soft leather might do fine.
I lay sandpaper on the 2x4 (400 grit cuts about right for touchup...300-320 would be fine, 200-240 is a bit coarse unless really reprofiling) and pull the knife edge into the paper and backing, towards my gut. Don't "cut into" the paper for convex, like you do on a flat bench stone. Pull edge towards you, like stropping. Takes some practice... I haven't mastered the super-hair-poppin edge yet myself, but can restore a shavin edge that scrapes skin flakes up also. Of course on a 12" Bowie that's fine. I'd like better on Sebbies and on Fallknivens.
Idea above from the original "soft backing material" courtesy of Cliff Stamp.
The Spyderco 204 will generally be harder to use in maintaining that nice convex edge. It might be possible, and I'll admit to being an "advanced beginner" on the 204 (I'm pretty good w/ a Lansky jig), but I managed to partly flatten out the convex final edge on my Sebenza on the 204 without regaining a super hair sharp edge.
With the 204, you'll have to stay slightly "tall" or obtuse on the angle to stay away from convex part of the bevel, and you may end up with a taller/blunter angle on that very final edge than you want. Maybe somebody out there will chime in if they have mastered convex on Sharpmaker.
