A perfect example of one of those is the lulzbot AO-101 lower X end piece. You can try doing a repair in netfabb before importing the stl to fix that, but usually if it won’t import at that point its because whoever made the stl had something odd going on in their file. If it doesn’t convert properly it might still be a surface at that point.
If it converts properly, it will be a solid gray object just like the other solids. You want the “make this into a solid” button. The “make this into a surface” button is the one that is selected by default. So what happens is when you right click on the imported stl file and select “convert to base feature” you get a little tiny window with 2 buttons on it. Heh, I was just looking for where that was to post it, glad you found it.
