Thank you guys for the quick reply
As you can see it's quite hard to understand where the bars are overlapping, so I generate these hatches that help me a lot in design mode..
As far as I understand the tool works this way: if I modify the polyline, it will be deleted and recreated with the resulting changes. (more/fewer bars, different orientation, and so on)
By now I was just generating hatches by selecting them but with every change of the polyline, I've to run again my old lisp.
The idea is to generate that hatch every time this polyline with that layer gets created. (and deleted of course)
The polyline (which is open, but it's not a problem) is in an independent layer.