Hi Igor - to do that, first select your object and run Edit/Separate on it to break it into individual surfaces.

That is necessary because to remove a trimming boundary any edge of the boundary cannnot be a joined edge between 2 different surfaces, which is what you will have in that case with the upper and lower caps part joined to the cylinder part.

Once you have the cylinder part as an individual object, select one of its edges, then do Ctrl+A to select all edges, then hit Delete. That will remove the boundary and erase the hole like you want.

If you still can't get that to work, could you please post the 3DM model file so I can take a look at it?

Basically, when you have a hole that crosses a seam like that, the seam is actually part of the outer trimming boundary. For example with a surface like this:




If you could look at the entire outer trimming boundary of that surface in UV space, you would see it looks like this:



So note there that the different portions of "seam" edges and also the top and bottom edge are all part of that outer trimming boundary in this case. I hope that may help explain what the full boundary is for this kind of case.

I would like to tune up the untrim mechanism in the future so that it could be smarter and not need to necessarily have the entire boundary selected to work, but for now you need to select all edges that make up a boundary for an untrim to work.

- Michael