Can't Use Slice Tool because object is not manifold solid

Hello, I am trying to use the slcier5 plugin however it gives me the error “object is not manifold solid”. I am using the Solid Inspector to close the holes but at some point it never becomes zero. I managed to bring it down to 3.

Can you help me solve this issue?

DAY 1 - Bench 2.skp (1.0 MB)

Work on getting rid of the internal face edges. I expect that will clear up the holes.

The structure of the object suggests that you built it from three sections that you joined together. That led to the issues solid inspector reports:

  • There is an interior face at each of the two joins. Their edges produce the interior face edges. Erase those faces and that issue goes away
  • There are slight mismatches between sections along the join. Those left holes that cause the surface border issues. You will need to redraw these areas to close the holes:

Thank you for your detailed answer, indeed, those suggestions solved the problem!