When I did my apprenticeship way back when, we made all our test pieces in the worst quality mild steel but every piece had to be mirror finished, by hand . Huge amount of time to go through every grade of emery. After two or three days you could see your reflection in it.
Flatting down GRP is the same when you want a mirror finish at the end. Wet n dry used wet, through every grade down to 2000 and you should be done.
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