With the edge square, I moved on to grinding the 25° primary bevel.
Most plane irons found in the wild will be ground higher than 25°. Even new Lee Valley irons are ground to 30°. The 25° primary bevel is important, so even on a new blade I take the time to grind the primary bevel back to 25°.
I put on a 40 grit blue zironia belt because I knew this iron was going to require a lot material removal. Then I used a block of wood to set the sander table to 25°.
To grind the iron, I wrapped my fingers from both hands around the top of the table and pinched the blade between my thumb, which is on the back of the blade, and my fingers, which were under the table. My thumbs guided the blade back and forth. The blade didn’t move as smoothly as I liked, so I applied a little paste wax to the table. When I focus on pinching the blade against the table, the grinding goes better.
My grinding has a bias. I tend to take more material off the left than the right side, so as I moved the blade across the belt, I tried to linger on the right side. When I started grinding the blade, I checked my progress after every pass. When the pattern become clear, I was a little more aggressive.
I checked to see if I’m grinding square by using a try square against the heel of the bevel.
Sometimes it is hard to tell which portion of the bevel is being ground by my work. When that happens, I darken the whole bevel with a permanent marker. One pass with the grinder will quickly show where I am removing metal.
When I felt my thumbs getting warm I stopped and cooled the blade by either dipping it in water or by pressing the back against a large cast iron table (jointer, bandsaw, etc.).
The grind does not have to be perfect. Just get close to straight and square. The picture above shows my ground iron. The witness line near the edge is because this is a laminated plane iron. The top part of the bevel is soft wrought iron. The bottom part is hard steel. I considered continuing to grind until the witness line was parallel to the edge, but the edge was reasonably square to one side, so I stopped.
While grinding and sharpening in general, it is easy to focus on the bevel and not actually pay attention to the edge. I stopped grinding this iron when there was only a tiny flat on the edge. You can see it reflecting light above. The last 10% of the grinding always seems to take 90% of the time. If I get impatient, I just go ahead and try the next step. If the next step takes too long, then I go back to the grinder. In this case the small flat was removed quickly during the next step.
After I finished sharpening this blade entirely, I found a nick in the edge. I suspect that nick was from grinding so close to the edge with the 40 grit belt. Next time I do a new blade, I’ll use 120 grit when I get close to the edge.