> Get a can of spray glue and glue some 50 grit sandpaper to a piece of
> 3/4 ply or other similar flat surface.

I would venture that 3/4" ply is anything but flat. I would use a piece of
1/4" plate glass or a pieced of machined steel or iron. Like a tablesaw
table or some such...

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mordo
1990 Carat