A little trick I learned when working at an air compressor company during high school. We would have to hand lap pistons on emery paper. If you went side to side, the natural pressure would sand more from the "sides" and less from the "top and bottom", going in a circle took more from the outside edge of the piston and less from the middle, as more pressure would be inadvertently put on the piston in the direction you were pushing it to sand it.
When hand lapping flat surfaces when the paper is stationary and you're moving the piece against it, do so in a figure 8 pattern to sand the entire surface evenly. Just figured this might help.
When hand lapping flat surfaces when the paper is stationary and you're moving the piece against it, do so in a figure 8 pattern to sand the entire surface evenly. Just figured this might help.