Hi Lars, it's pretty hard to see what might have gone wrong just from looking at an image... Can you please post the .3dm model file of the part that did not work right or send it to me at moi@moi3d.com ? That will let me take a look at the actual model shapes and see if I can give any information.

From looking at your image, it kind of looks like your original shape was a single surface, which shouldn't normally do anything different with Separate or Join on it since it is just a single piece, unless you've actually got 2 surfaces there stacked up on top of each other, which if so would be the problem.

- Michael