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You need to re-season it. Follow the instructions given above... but make sure the linseed oil you use is pure food grade and not the hardware store type that has other chemicals in it.
Linseed oil is not toxic, in fact it's good for you. But make sure the oil you used was pure oil and not the kind you get at the hardware store.
The pitting was most likely caused by rust, not the brushes. The brushes just uncovered it.
500,000 volt Van de Graaff Generator Using Cheap Parts
Definitely like that copper tubing idea to mitigate leakage from the bottom of the sphere. When I made one of these, I never did solve that problem and ended up putting a bigger motor on the generator to generate charge faster than it could leak away.