I know this sounds ghetto, but has anyone ever heard of a kit that was sold that had adhesive sandpaper forms that you stuck on a valve lip and then chucked in your drill and let it lap/grind the valve seats? I think it was JC whitney about 20 years ago? Maybe the drill acted on the valve from the chamber side with a suction cup? Vaguely remembered something like that. Like an accelerated lapping job.
