Re: Gear case lapping question

[mr-asa]

As Collector said, lapping doesn’t involve sandpaper.
When I do it, I get a piece of 1/4" thick glass (at a minimum) that’s about a foot square, then I just use valve-lapping compound like you’d find at any auto parts store.

With a flat enough surface you shouldn’t need the sharpie to see the high and low spots after a few passes.