Wayne is right. If you use glue that doesn't dissolve the paint then you are only gluing to the paint so your joint is only as stong as the paint. The best way is to scrape a small area for the joint, and touch up the paint as necessary after joining.
Tenax 7R works pretty well because it is so thin. It will dissolve the paint since it actually melts the plastic, but it is so thin that it gets pulled into joints and doesn't do much damage to the paint outside the joint.