Just flux. A bit of solder, and hold to flame. Melts and flows rather well. Got very good results fitting bearings to w iron springing. If you want a video, Ill try to record one and post to youtube. I have an hour or so free tonight.
Admittedly its not for building kits, but soldering corners for strength, sweating parts together, or just doing something where an iron wont go. Fire is your friend.