This can be a very slow and tedious process, especially on an old iron that was never properly flattened. You will probably find at least one or both corners dubbed over. If the corners are too badly rounded over you may have to go back to the pedestal grinder to grind the bevel down to remove the rounded over corners. Keep the lapping plate wet with water. You can add a couple drops of Dawn dish detergent to the water. This acts as a lubricant and helps keep the swarf in suspension.
When the back appears flat across the entire cutting edge move on to the next grit which is blue. This time swing the angle you hold the iron to the plate in the opposite direction. This makes it easy to see when you have removed all the scratches from the previous grit plate. Work your way through the red and green plates each time swinging the angle to the plate in the opposite direction and being sure to remove all the scratches from the previous grit. When done with the 4 grits of diamond plates I move on to 4000 and 8000 grit water stones being sure to flatten each stone before use.
Finally! The back is flat and polished.
When you are finished the back of your iron should look like the photo above from corner to corner across the entire cutting edge. This is a tedious, time consuming job, but it only has to be done once.
The setup for lapping the micro bevel.
The photo above shows how I set the iron in the honing guide to lap the bevel. These digital protractors are accurate, repeatable, and inexpensive. I lap the bevel at 30ļ creating a micro bevel. The purpose of this is to reduce sharpening time. Some people create 2 micro bevels. One for the medium stone and one for the fine stone, but I havenít found this method to save any more time so I hone on the 4000 grit stone and the 8000 grit stone at the same 30ļ setting.
When developing the micro bevel I also create a slight camber on the cutting edge. To do this place the cutting edge at the top of the 4000 grit stone. Place your left index finger on the left corner of the cutting edge. Place your right index finger on top of your left finger and while applying moderate pressure from both fingers draw the iron towards you for as complete a stroke as you can without letting the guide wheel drop off of the stone.
Do 4 or 5 strokes this way. Then move both fingers to a point that is midway between the corner and the center of the iron and do 2 to 3 strokes with moderate pressure. Then move both fingers to the center of the cutting edge and do 2 strokes using moderate pressure. Repeat the previous 2 steps on the right side of the iron. You should now see a shiny surface at the cutting edge that is slightly wider at the corners than at the center. If not repeat the steps above until you have the desired micro bevel.
When you have a micro bevel that is wider at the corners tapering to narrower at the center you can take a couple of strokes to blend the camber. To do this place the cutting edge at the top of the stone and place your index fingers one at each corner of the cutting edge. While putting moderate pressure on the left corner of the iron and almost no pressure on the right corner begin to draw the iron towards you while at the same time decreasing pressure on the left corner and increasing pressure on the right corner until you have moderate pressure on the right corner and almost no pressure on the left. Then take another stroke doing just the opposite.
Start with pressure on the right and gradually shift that pressure to the left. This step tends to fair the camber a little. Now you can repeat the above steps using an 8000 grit stone. After which your iron is ready for use. The purpose of this slight camber is to prevent plane tracks when planning surfaces that require multiple, overlapping strokes. Be sure to flatten your water stones prior to use.
This is the setup I use to hone the mating face of the cap iron.
This photo shows the slight angle that ensures
contact at the tip of the capiron.
Next the capiron must be fitted to the iron. This is the second but equally important reason for flattening the back of the iron. When the capiron is assembled to the iron there cannot be any gaps in the contact area. Even the slightest gap here will allow shavings to get stuck and choke the plane. There must be complete contact between the iron and capiron and that contact must be at the very outer edge of the capiron.