Hi Dimitri, sorry when you talked about the boolean I was thinking you were talking about booleaning the lid onto the body, I guess because those looked like the 2 objects that were touching each other in the rendering. When you have a question about how something is behaving it can help a lot if you post the 3DM model file instead of only screenshots or renderings, it helps a lot to clarify things when I'm able to examine the actual model.

So for the boolean I should have described a bit more - if 2 objects like 2 mirrored halves are very precisely aligned along a common plane where they touch then usually that can be booleaned. If they're at a very slight angle to each other that can be a problem though. If you can post your 3DM model file I can try to inspect it closely and I might be able to give you some better information on what's going on there.

It is also possible to unify a mirrored object by using the Edit > Join command instead of booleans. That can be successful in cases where booleans fail because the Join command only tries to glue unattached edges to each other, it does not try to do any intersections between the objects like booleans will do. So to use that method you would delete any plane running down the mirror line first so that the object had open edges there that were not already joined between 2 surfaces. Then after the mirror using Edit > Join will glue those together into a unified object.

- Michael