Has anyone a good way to clean the rough-cast grey area in the spokes?  I want to paint that space Musou Black to see how it looks; but want things clean prior.

Sandblaster.......no I don't know, worst zinc plating ever, I rode 8 hours in the rain on a 2006 WG one night, shift work so didn't get back to it until the following evening, fucking spokes was covered in surface rust, pitiful for the price.

Sorry, your pic didn't load, there are some top notch mag cleaner stuff out there, some more aggressive than others, still can't help..

I'd reckon that a good going over with a stiff scrubbing brush and plenty of prepwash wax and grease remover would be all you would need to do, the cast surface is relatively rough so there is plenty for the paint to key into.