I guess what I'm talking about. Can you make a Van Der Graaff that works in the opposite? One that charges a plastic ball instead of aluminum or steel? but does it already do that? The wand? I'm pretty much incompetent when it comes to this.
Most Van de Graaff generators charge positive but they can also be build so that they will charge negative. When a grounded metal object is close to a positively charged VDG it will become negatively charged. Plastic objects don't conduct electricity and so they can't be charged up that easily. But you can charge a plastic ball by rubbing it.
Attaching it to the opposite brush will work too.
A rubber ball can not zap you. To get a shock all the electric charge in the ball would have to flow into you within a tiny fraction of a second. That can only happen when the ball is a good conductor.
But you could spray the ball with copper or graphite spray to make it conductive.
Sure, you can paint it. It will still work.
When the aluminum sphere is bigger it will store more positive charge. Whether the negative charge on the grounded object changes depends on different factors.
As long as the grounded object is not too close to the aluminum sphere it's charge is approximately proportional to (radius of the aluminum sphere) / (distance from the aluminum sphere).
If it's very close to the sphere it gets more complicated and the amount of charge depends mostly on the distance since the distance determines how high the voltage can be before a spark jumps over.