Boolean difference problem - solid becomes joined surface

 From:  Yenmonger (OTTERMAN)
Gentlemen, I seek your counsel. I'm making a spaceship miniature for rapid prototyping, and have begun putting on panel lines. I do this by projecting a line onto the solid, then doing a sweep of a wedge shape along the separated projection lines. The wedge shape will help with the molding and casting process, and is good at 'catching paint'.

However, when I try to diff one of the sweeps from the main body, my solid turns into a joined surface (in some cases, worse!). I've tried scaling the swept object, and some repositioning, but nothing seems to give me a clean diff.

Any hints?