The inner wall of the square tube can not be coated with electrophoresis.

I have been drawing cars for more than five years. Your problem is simple and can be solved.

There are the following schemes. You try.

1. Can we open more process holes in the pipe?

2. Are you cathodic electrophoresis or anodic electrophoresis? Compared with cathodic electrophoresis, spray painting is much better. You can use a wire to connect one end to the outside of the pipeline and the other end to the inside of the pipeline (you can choose more connection points), which is equivalent to adjusting the internal and external potentials to the same height. This painting is very good. I think you can try this method.

3. If you think that the second scheme will make electrophoresis of inner wall connection points impossible. You can put the anode point in the middle of the inner wall with wires, so that the inner wall can be brushed without touching.

If you are not clear, please add me QQ26099489 or email me to lizh@landwind.com, and I can draw a schematic diagram for you.