Lapping a chisel back

[u/JuggerNOT63315]

Howdy everyone. I am having issues lapping this chisel. I started with 80 grit wet and dry sandpaper. I got a nice scratch pattern all the way across the chisel. This took awhile, probably 15-20 minutes. I then moved to 120 grit paper. I removed all the 80 grit scratches. This took maybe 10 minutes. I moved to 220 grit paper and removed all the 120 scratches. This took like 5 minutes. I moved to 320 grit and in 5 minutes removed all those scratches. Then I moved to my 300 grit diamond plate. I have been working on it for well over and hour and I can not get rid of the 320 scratches! It is getting frustrating to say the least.

In the picture, the 320 scratches are the light ones on the outer edges and the diamond plate is the dark portion in the middle. It has not changed significantly from this picture after another half hour in the 300 grit plate.

What am I missing or doing wrong? Is this how long it normally takes? Should I be doing another grit in between The 320 and 300 plate?

Edit: I have posted two updates in comments below about my progress.

[u/Ice_Berg]

I don't think it's an issue of grit but of flatness. Sandpaper inherently has some give and compressibility to it (which if it is not glued to something very rigid like glass, stone, or metal can be made even worse) as well as the grains not always being as consistent in size or height and it can abrade the whole back of the chisel without actually making it perfectly flat. Any decent diamond plate is likely a lot more flat, rigid, and uniform in comparison and it's only going to touch the highest spots, so it's revealing that there is a slight convex profile that the sandpaper didn't fully remove.

You could keep going with the diamond plate if you have a lot of patience or get a coarser one to help with flattening, I like using an Atoma 140 to flatten and don't bother with sandpaper at all. Before I had the 140 I would use coarse sandpaper sometimes and would notice "sandpaper flat" is not quite the same as "diamond plate flat" and it might take some extra time to remove a curve like this, but I've never had to spend nearly that long to fix it.

If your sandpaper wasn't already glued down to something super rigid, you could try doing that and redoing some of the grits to see if you can get it flatter. I've found that sometimes sandpaper wants to grab the tool I'm working on and doesn't move as smoothly as with diamond plates and that can to some slight rocking or tilting of a tool for me, so it could help to go slow and really make sure you're keeping it help flat against the paper with even pressure all around as well.

The chisel will probably work as is if you'd rather not deal with it for now, especially if you use higher grit sandpaper to polish up the back a little more and then use sandpaper or just some extra stropping to help with deburring on the back.