Hi,
1st you need to locate the bare edges on the object.
I use the attached script: SelectNackedEdges.js (I put the F10 shortcut); See the attached images.
secondly, I deleted the surfaces in question.
Then I redid the surfaces (in green in the attached file). I joined the surfaces together two by two and finally joined them with the object.
Well, I'm not sure that's exactly how I did it, as I had to make several attempts before I succeeded!
Michael must have made it simpler and quicker.