I usually start with a flat grind on a glass platen, leaving around .040" or so at the edge. I bring the grind up the blade where I want it, but only go to 80 or 120 grit on the belts, mostly to make sure the 36 grit scratches are fully out.
I have a piece of S2-32 hard felt 1/4" thick double sides taped onto a metal platen, as well as a piece of F3 felt (a bit softer than S2-32, also 1/4" thick) double sided taped to another metal platen. I also have some 1/8" thick leather on another metal platen. Each of these has it's place for convex grinds.
For heavier chopper knives, I use the F3 more since it will deflect farther with less pressure than the S2-32 felt, leave a more rounded convex and thicker behind the edge for heavier tasks. For kitchen knives and thin scalpely blades, I use the S2-32 mostly. The S2-32 gives a flatter convex further up the blade. The leather is to help keep the plunge cuts more crisp and clean up any deeper scratches since it barely has any give to it. It's pretty close to the S2-32, but will keep the corners sharper in the plunge. The corners of the soft platens bend and get smooshed down quicker than the face does because the pressure is on a smaller area.
I run the belts slower because running them fast will burn the felt or leather. I often start with the edge up when it's still thick, but as it gets thinner, I switch to edge down for safety or run the belt in reverse and use the edge up. I plan on leaving some width at the spine, so if I get a full convex that hits the spine as well, I can trim the spine back down to the full thickness I want, or buy thicker stock to begin with. With the soft platens, as you push into them, you will feel the felt/leather give a bit. You can actually establish a groove in the felt or leather that will help keep the knife there and consistent, too. The belts will be less effective with the give to the backer, so using fresh, sharp belts is key! Trying to use a worn belt on a soft felt platen will not do much on a hardened blade! It's also very easy to put too much pressure on one side or the other of the platen, too, so you really need to feel your pressure on the blade to get a nice grind without ripples in it. Keep checking down the blade to make sure you aren't putting edge to spine dips/ripples into the blade. Too much pressure can cause the belt splice to put a "worm trail" (squiggly ugly grind line) on the side of the blade mostly parallel to the edge.
With the felt, it's all about the feel of the blade and felt. I can feel more where I am grinding and know to push more towards the edge to thin it out, towards the middle or towards the spine to change how the convexing is. I am working on a kitchen knife currently from 1/8" thick stock and the customer wants the spine at 1/8" and a high convex. First grind I did, I could feel the convex shoulder about halfway up the blade parallel to the edge. It cut some stuff OK, but not onions. Went back to the felt platens, applied pressure more at the midpoint on the blade height wise to push to convex towards the spine more, especially on the front half of the blade and then pushed the area just behind the edge into the platen more to thin out the area behind the edge. Made a huge difference in how it cuts!
With soft platens, its hard to keep crisp grind lines and plunges. Doing a convex hunter/EDC type blade for a full height grind isn't too bad except for the plunges. Kitchen knives need to perform well in a variety of foods, so the grind is more important there. I grind, test, grind, test, grind, test until I am happy with how they perform. With convex grinds, many tend to go partway up the blade, but this will leave the tip portion of the blade thicker, which I found, so I have to work on pushing the convex up the blade towards the spine as I get closer to the tip, or grind in a distal taper in the blade itself to help out with it before grinding the bevels.
Gyuto started: Flat grinds
Blade is oversized (see sharpie lines), but convex being put in where the flat bevels were, bringing them closer to the spine:
Choil shot after first grind:
Choil after 2nd grind:
These are both using the S2-32 felt platen. I can still get a strong convexing with the firmer platen!
Not much visible change, but I thinned out the area just behind the edge a bit and worked the grind up as I got closer to the tip, so the choil itself didn't see a big change! The shoulder of the convex is much closer to spine on the blade, especially towards the tip. Leaving the blade oversized lets me trim the spine down and still have full thickness, especially since the barstock is oversized already due to mill scale. If you use precision ground stock or stock right on dimensions, you may need to play a bit more to keep the spine the thickness you want.
For contrast, here is a 3/32" Magnacut gyuto choil:
Same S2-32 felt platen, 3/32" magnacut and really pushing hard into the felt and bringing the grind way up. It also has a S grind (36" radius platen hollow grind), but it's not visible from the choil since I start it forward of the heel so there is a fatter heel vertical edge to round out for comfort. This blade also has more of a distal taper as well and I didn't worry about keeping the spine full width because I wanted a laser.
With a rotary platen, I would start with the flat grind (or faceted flat grind using slightly different angles and then regrinding starting with 60 or 80 grit belts to blend those on the rotary. With rotary platen or felt, you will see the scratches that you missed; keep going until all of the scratches are getting removed evenly along the grind. If you see deep scratches in an area of the convex, that means that area isn't really fully ground yet to be even with the rest of the blade! The rotary platen uses a rubber belt to give tension behind the belt to vary how much give there is, sometimes with tension adjustments or space between the rubber wheels. The belt can give especially on the edges, so even pressure is important. The rotary platen helps keep plunges crisper than the soft platens with practice. The rubber belt can cup as well, giving a 2" groove in the blade, so that is one thing to consider!
I saw someone else used a set up with 2 rubber contact wheels offset vertically to mimic a rotary platen, but with no rubber belt behind the main sanding belt.
I can try to post more info later
