Derek Cohen (in Perth, Australia)
Hi Charles

A couple of points:

Firstly, while it is desirable to have a flat back, it is really only necessary to have one that is coplanar. That is, has points along the back that are level, starting at the edge and ending as high up the blade as possible.

Secondly, only the first 1" at the back of the blade needs to be polished. Actually, only 1mm needs to be polished, but 1" is easier to do. Remember, the grit you end with on the bevel face should also be used on the blade back.

Thirdly, I used to flatten blades on sandpaper on glass, but now prefer to do so with diamond paste on cast iron. The problem with sandpaper is that it wears, and where it wears hollows develop. This can then cause the steel to abrade unevenly. At least cast iron does not wear this way. Either way, the rule is that the next grit must remove the scratches from the previous grit, that it must do so within a few strokes - otherwise you are moving up the grit chain too fast, that is, have skipped an important grit. My rule-of-thumb is to "double up" (or close to in what I can get). I never use coarser than 120 grit to start (as the scratches will be too deep), then go to 240, 400, 600 and stop at 1200 before moving on to waterstones. With diamond paste I would start at 40 micron, go to 15 micron, then 6, 3, 1.

Four. You have to keep the blade flat as you move it. Any rocking with create a camber. To hold it flat (on the media of your choice) I began using a magnet several years ago.

To keep track of progress it is advisable that one marks the area with a permanent marker. This will help one see where are the high and low spots.

This was a blade at 0.5 microns. Look for any distortion in the reflection. This looked OK to my eye (but what do others see?!) ..

Regards from Perth

Derek