Hi Michael,

thank you for you answer, explanation and suggestion.

But I am still curios:
I am designing the model, adding details, checking if the solid is "healthy".
In one step, adding a detail the solid is "broken" to a joined surface... despite the operation does NOT affect the areas which are now marked as naked edges.
And separating the joined surfaces and re-joining them creates a solid again(!) - which is not "healthy" anymore (i can identify it only by a next boolean operation which fails, and the stl exported from that solid cause the slicer is crashing) So it means there is something wrong with faces... Is there a way how to identify it and prevent such situation?

Is there an the explanation why in some cases the "boolean Union" end up as Boolean Difference? , and the similar object with same geometry (being a joined surface) in boolean union with solid ends up as a healthy solid (allowing subsequent operations)?

I don't want to bother you and waste your time, if my questions are obsolete and I am alone asking such things, just leave them... :-) . I will take more care at design...