>>Isolate these objects by right-clicking on Edit > Hide so they are the only visible objects:

I NEVER made a right click on Hide button to isolate... but this does the trick - i get nacked edges

Thanks a lot!

>It really is a shame to work with data like that though - an STL file like would likely be coming from a CAD program and it's so much better to load in the original CAD data using STEP format for example.
YES i agree but in the 3D printing scene often you have only a STL file to work with..

And now i have to right click all the buttons in MoI and see what happens... ;-)