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The back of my mortice chisel is not completely flat. Specifically, the corners of the cutting edge are both slightly shallower than the rest of the back face, meaning they aren't honed to form a sharp edge. Here you can see the remnants of permanent marker left after dragging it across a fine diamond stone.

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Here's my process for getting the back flat (it is essentially this method from Paul Sellers but with plate glass instead of a granite block):

  • Stick wet and dry sandpaper of 120, 180, 240 and 320 grit to a piece of plate glass
  • Wet the sandpaper using glass cleaner (it's just what I usually have to hand instead of water)

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  • Drag the chisel backwards over each grit of sandpaper a few hundreds times, working my way up through the grits
  • Move to fine, then extra fine, DMT diamond stones

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No matter how hard I try, I cannot get those corners to be flat with the rest of the back. What am I doing wrong?

About the plate glass I have checked that the glass is flat using a straight edge (manufacturer claims tolerance is 0.042mm over 100mm). Perhaps the glass isn't flat.

It is supported from below with rubber strips glued to almost all of the underside

About the chisel It's a Narex 12mm Mortice chisel

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    Stick with the coarsest grit until the back is flat, then move on to the finer grits. Don't try to flatten the entire back; the last half-inch or so is all you need. Commented Mar 2 at 23:03

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In short it's because once you get to nearly flat the amount of metal that needs to be removed becomes greater and greater, which can enormously impact the speed of material removal — because you're aiming to flatten a huge swath of the back of this chisel think about the area of steel you're attempting to abrade.

Although it's hotly debated in woodworking circles — and always will be :-) — there's generally no functional reason to completely flatten the back of any chisel, zero. This is including paring chisels and mortise chisels even though they are often held up as specific examples of where it's not just desirable but necessary to get their backs flat. The necessity for this is proved false by examining vintage paring and mortise chisels that were used by craftsmen of previous generations..... although this simple fact does nothing to stifle the debate.

Much related info in one of my previous Answers, How to flatten stubborn hollow on the back of a plane iron?

You're also not really using the ideal process or material for removing lots of steel.

Drag the chisel backwards over each grit of sandpaper a few hundreds times, working my way up through the grits

Note first this is an inefficient way to remove lots of material anyway.

And standard wet-and-dry is often not really up to this, being too friable/easily worn down. High-end paper/film will do better and last longer, but (debatably) the way to do this for most of us is simply to get an extremely coarse diamond plate. Not only will this abrade literally anything in the shop, including carbide, compared to most affordable options it's practically immortal. I've used an inexpensive 150 diamond plate for a few years now (regularly wishing I had an even coarser one BTW) and it seems to still work just as well as it did after the initial break-in period was over.

Additionally there's an inherent mistake here as well — moving up the grits too soon. Actually two, since it seems you're seeking to polish that whole swath of flattened back. Firstly you don't go up the grits until you're done, i.e. when the coarsest grit has done all the heavy lifting. And secondly, why polish the whole of back anyway?

Only the portion of steel right at the edge actually needs to be taken up through the grits since the edge is, of course, the only part that does any cutting :-) As mentioned in the above link I aim for only the narrowest strip to be dead flat and polished and the amount of time this can save, even on relatively soft vintage plane irons, is, ah, 'considerable' (massive understatement — on tougher steels it could be mere minutes versus hours).

In support of this being enough for function, a fine hone on only a small portion of the back of a chisel or plane iron is nearly always what is observed on used vintage chisels/plane irons. And rarely, very rarely, will you find a chisel that was flattened over a significant length of the tool — significant numbers of older chisels retain their original slight camber, where they curve slightly towards the bevel side (this curvature could be introduced at quenching and we should take note that it was deliberately not ground out at the factory).

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    In fact, the "only flatten the back of the actual working edge" observation leads directly to the Ruler Sharpening Trick (websearch that phrase) which is a way of forcing yourself to consistently do only that. Done folks like this trick, some loathe it, but it does work.
    – keshlam
    Commented Mar 28 at 19:50

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