In electrostatic paint spraying, after one coat of paint is attracted, paint is no longer attracted to the frame. Explain why.(For example take a...

This is really basic physics and an understanding of atomic parts.

Electrons flow from the positive side of a battery to a negative side.  So if we put a space between them then the electrons do not flow unless they arc.

So now comes the spray gun.  It provides a positive charge to the paint particles and then the air pushes the charged particle toward the negatively charged frame.  As soon as the positively charges paint particles touch the negatively charged frame, the charge on the paint particles is removed and the paint layer acts like an insulator thus lowering the potential difference between the spray gun and the frame.

As the frame moves down the assembly line, there are other frames that are have a larger differential in charge and the paint goes toward those areas. 

As the air blows the paint toward the frame, the tubing causes the wind to curve the paint around the frame where the tubing is not yet painted and thus exposed.  That exposed surface attracts the paint until it is covered and then the paint acts like a insulator and the differential in charge is lower than any other bare surface so the paint no longer attaches to painted section.

That is how it works.