I unioned the 'cupola' which was built by a revolve, and then squished with a scale, to the body, and besides the trim line where the 2 join, there is a loop that goes down into the body...



also, when I click on the cupola, it appears there are two surfaces. But I can't drag or get rid of one of them. If I select it, and hit delete, the surface totally disappears...



Okay, wait, I just got it to work. I deleted one of the surfaces, and it looks like we got funky geometry on the inside....



I also attached the 3dm file, where the two pieces haven't been unioned yet, so you can try it yourself...


Hmm, I saved the file with the non-unioned pieces, and when I tried to reload it and do the union, MOI crashed. I did this because it seems that revolve sometimes doesn't generate a closed surface, until the file is saved and reloaded...

Okay, tried it again, and it crashed again.