When I'm trying to take grease pencil off something, I start by setting it in the sun to heat up. Then I scrape off as much of the stuff as I can with something stiff but porous. Lots of times it's a big chunk of wood mulch or a stick or a popsicle stick. Then I let it warm up in the sun again and use a paste of hot water and baking soda to clean the surface.
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