yes, I think most ideally it would be positively charged. Like what you said, if it is negatively charged, the positively charges (don't write proton in Physics, you will get marked wrong) of the body will eventually "run out" and if the painting is not complete then yeah, the droplets would not be attracted to the non painted sides of the body.

HI XUAN YU yes electrons will still be transferred from one material to another of the same material as long as the materials are insulators. Not required to know in examination though.