Originally Posted by
Euclid440
If you do have a frown, (blade width narrower in the middle than the heel and toe), you must reprofile the edge straight, so the blade width is the same from heel to toe.
To do so, you grind the edge straight down, 90 degrees to the stone or diamond plate, usually in a bread knifing sawing motion or high angle grinding, spine off the stone near 90 degree until the blade width is even. You can stroke backwards or forwards, it is not pretty, you are just hogging off material. It does not matter which technique you use, exactly the same amount of material must be removed to get to a straight edge.
It is best to take measurements and mark the blade width at the narrowest point and draw a line with a Sharpie, so the heel and toe match the middle and the blade edge matches the spine. Sneak up on the line as you grind, and stop and take measurements frequently, so you remove the minimum amount of steel. You can buy a pair of plastic calipers for a few dollars, a good investment.
If you just start grinding willy nilly you will remove more steel than you need to, having to correct for removing too much at one point. Take the time to measure and draw straight guideline. If the spine is curved, it probably had a smile to begin with.
You also should carefully examine the razor to identify, if there is an issue that caused the frown, a misshaped heel or high stabilizer, heavy straight stroke honing or a stone that is not flat are common causes. Then repair the razor and pay attention to not repeat the same error that caused the issue.