Thanks Michael for your explanation.

I still don't understand why some combination of solids wont union, event worst, in some cases it goes bazurk, I know you need a file and here it is.

It's just a test I did trying to find something that wont union and I got even better, union went bazurk on me. Basically, I created a sphere and used a plain circle to create a ring of sphere that union without problem. Then I copied the first ring and I put a few on the outside of the first ring almost randomly. I tried to union all 6 rings together and voila! A joint surface was created and a bunch of unjoined surfaces where created as well but I tried to join this result and it worked, creating a single object but you'll see as well that some shere have lost some part of there skin sort of speak.

Since it's not the first time I obtain a similar result, I'd like to know if there a way to at least get a correct joined surface, by this I mean without lost of some part of the original surface. Though I think I could create a good joined surface in this particular case I would appreciate this a lot since I would use this method to create the central part of flowers with a few hundred tiny sphere disposed in spiral on the surface of a dome for example, sorry I don't know the name in english.

Thanks,
Felix