Chisel backs getting out of flat with sharpening over time

[Fitzhugh Freeman]

I'm sure this has been covered but I failed to find it in searching.
I have a problem with convex areas just behind the edge of chisels. I think it's from my technique in flattening the backs and exasperated by how I sharpen. I want to fix the ones I've already done this to, and I just received some new chisels and don't want to mess them up too.

I just got a new mortise chisel and a few new to me bevel edge chisels and I want to avoid causing the same problem with them

I've read arguments against having to fully flatten the backs of any but pairing chisels and don't want to get into that. I run into problems with the backs not being flat since I often use the backs as registration surfaces, same reason I saw given for pairing chisels. I only own a few and they all get used for pairing at times. I don't own bullnose, shoulder etc. planes so I use chisels for pairing in cases where I might not otherwise, so even my super cheap orange handled chisels get a lot of use pairing.

All this applies whether first sharpening or just removing wire edge, and the effect is small but there. I read that you only have to flatten the last section (some say half inch, others 2 inches and so forth). Unless I slide the blade side to side the problem occurs. I find it much easier to slide forward and back - perhaps that is the source of my problem? The drawings are obviously highly exagerated...

chisel_bevel_1s.png
When I flatten the back I move the chisel forward and back. This means the part that gets the most work is the front area, as marked. It ends up making a bit of a concavity.



chisel_bevel_2s.png
Only, as I move it forward and back the rear section also ends up on the flat stone (glass w/ paper in my case).




chisel_bevel_3s.png
This leads to a rounding of the front, as seen above. Lines show how flat surface as blade moves forward and back. The blade itself is tipping, not glass, of course.




chisel_bevel_4s.png
Which causes the above dreaded convexity just behind the tip.





So, two questions: How to prevent this? And how to fix it?

I think removing the wire edge should involve removing little to no material from the back itself so I suspect my chisels have been like this since I first flattened them and I've only become aware over time.

I think I should invest in a steel plate and some carborundum powder. Are the plates milled or ground super flat and what type/hardness steel should they be? Seems getting some flat ground annealed steel plate would be much cheaper than a similar piece of metal sold for this purpose. For the prices I've seen I could get three pieces and use them to flatten each other. Heck, always wanted to try learning hand scraping... (too many other projects on the list already).

Thanks!