lapping in a model IC engine valve and valve seat

lapping in valves for a model IC engine is part of the "black art" of making working model engines which I attempt to explain here. I'm doing the lapping before installing the valve guides in the engine in case some of the valve and guide pairs don't work out. once I have 16 good pairs I can proceed with installation.
what I forgot to explain in the video is that while there's not yet a port drilled into the side of the valve cage, the valve stem and the valve guide have .002 inch clearance where the air will escape fairly unimpeded if the valve seat isn't sealing. if the seat is leaking then you can often hear and feel the air coming out around the stem.
and in case I wasn't clear enough, I'm using "Brasso" metal polish to do the lapping.