Martin, WOW!
It works! You live - and learn (c))))

Thank you very much!

Martin, I hurried with delight, alas.
In simple objects this metod works. But some - why do not.
Here is an example: try to remove all the bolts and "repaired" hole from them.

Hi krass - that one in your post above doesn't work right because there is an extremely thin little sliver of an additional face in that spot, making a somewhat more complex edge structure in that area than you would initially think.

If you do some careful window selections (selection window drag from the left going towards the right so only things completely contained inside the window are selected), and delete the tiny thin sliver face then it should work better for you. I've attached a version where it should be ready to have the holes deleted now more easily.

If you need to remove a hole where the hole goes across several surfaces, it's a more difficult process to remove those, you typically need to do a broader untrim and then retrim after that. There is an object repair tutorial here which goes over the process: http://moi3d.com/forum/index.php?webtag=MOI&msg=446.17

- Michael

Michael, thank you very much for your advice and for what spend your valuable time on my problem! I will study your recommendations, thank you!

There is one thing to add here.. Although normally you don't work Booleans on "surfaces" (Mostly left to trim/join/separate") At the step of "I joined surfaces", if you do a Boolean add, that will create that face without the trimmed surface, doing a merge process too....

FYI