Using a popsicle stick, or other small implement, dip into the lapping compound jar and bring up a small dab of grit. Spread it on a blade; try to get it on the outermost surface. Working carefully, put a fine coating of grit on each of the blades. The grit has to be spread along the entire blade, right out to the left and right edges of the blade. Once I failed to get the grit out to the edge, and the mower made a snick-snick sound for months until I re-sharpened. You don’t need to put grit on the painted flat side of the blade where it attaches to the spider; this surface does not come in contact with the cutting bar.The next step is adjust the cutting bar to bring it very slightly closer to the rotating blades. This is where you take into account what you learned with the slips of paper earlier - if all you had found earlier was tight spots, you might not even need to move the cutting bar. However, usually there are one or more spots where the paper failed to be cut, and so you would adjust the cutting bar.
The cutting bar has a swivel point and an adjusting system on the left and right sides of the mower. You want to adjust each side of the cutting bar the same amount, so as not to introduce a twist in the cutting bar. On the Craftsman, there are two screws that govern the cutting bar position. To bring the cutting bar closer to the blades, you would loosen the forward screw and tighten the rear screw (the screw nearest the rollers). How much do you turn the screws? Not much! If you think of the top of the screw as a clock face, you probably want to move the blade of the screwdriver in increments of about half an hour - which would be 1/24th of a revolution. First, loosen the screw in front 1/24th, then tighten the screw in back 1/24th. Do the same on the other side.
[Another common adjusting system uses two nuts on a threaded rod. The principle is the same: loosen one nut a tiny fraction, then tighten the other nut the same fraction. Repeat for the other side.]Now, test the blade rotation. It should be harder to rotate the cutting blades, but not impossible. If it is impossible, fiddle with the cutting bar adjusters to loosen the interference. You want some grinding to go on, since that is what will bring everything to a consistent clearance. But you don’t want free-spinning, since no sharpening occurs then.
Push the mower to cause the lapping action
This is the fun part. Flip the handle over, and start pushing the mower backwards. The blades should start spinning grudgingly, making a gritty kind of noise. Don’t go too fast. You don’t want the grit to be flung off the blades. What I do is go up the sidewalk 100 feet (two houses), turn around and push it back, grinding all the way. When I get back I evaluate the ease of spinning: if it has gotten too loose and easy I decide whether to adjust the cutting bar again and make it slightly tighter. It usually takes a couple of 200 foot round trips before I need to think about tightening. If your mower only needs a little bit of sharpening, as evidenced by the paper cutting test, one or two cutting bar micro-adjustments are all you need. If you know the mower was way out of whack, then two or three adjustments may be necessary.
Your ears are your most important tool in this phase. When you start out you will be able to hear the blades that are out of alignment as they pass over the cutting bar. You’ll hear snicks among the gritty whirring. As the lapping compound does its work you’ll hear fewer, softer snicks, and more of a steady, gritty whir. When all you hear is a steady, gritty whir, and the effort to push it has gone down markedly, you can stop pushing it up and down the sidewalk.
