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In my own endeavours I've used 240grit (220's near enough), 400 and 600 but only one of those at a time... depended on what was laying around.

You 'probably' dont need to worry about multiple...

I'm lucky to have access to a large cast iron surface plate at work.

When i got my norton perfectly flat on the 8K side it stuck to the plate so hard that i had to push it off the edge to break...