Glue a sheet of 400 sand-paper onto a piece of glass.  I use spray
glue, a minimal amount, making certain there's no foreign matter that
might be trapped in.

Holding the slide valves and applying a minimum pressure at the center,
and two other fingers to do the pushing, sand the surface in a figure-8
pattern.  Machinists use this technique to get a flat surface.

These days spray graphite is available, and will provide the best
manner of application.

Tom Binnall
Seattle