To clean up a gunked up freewheel -
Hose it with WD-40
Take a welder's wire brush, and scrub around it. the long, stiff metal bristles clean deep down between the cogs
take a rag, insert it into the spaces between the cogs, and shoeshine it.
It's messy, but easy. It will end up looking like new.
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"Love is not the dying moan of a distant violin, it’s the triumphant twang of a bedspring."
S. J. Perelman