Galoots

I bought a huge wooden panel raiser plane with a 4-1/4" wide tapered
iron, and started flattening the back of the iron on my coarse diamond
stone. Five hours later, and only about half the cutting edge is flat
(the two corners are low). I've never dealt with such a wide blade
before, but there's got to a better way.

So what's the better way?

The smart thing to do is to put a back bevel on the iron and call it a
day (like the last guy who owned the plane), but I now consider this a
challenge and want it the back flat.

Would a belt sander work? Or would it round over the back of the iron?
Or would it destroy the temper of my iron in 4.7 nano-seconds? I'll
keep dipping it in water.

Thank you for your responses in advance,
Chris