Are you talking about welded frames and not lugged frames? if so I back in the day we used bondo on the joints when customizing motorcycle frames, you can make them look flawless, looks just like fillet brazed joints when finished, and very lightweight. Just be sure the the area you want smoothed is bare metal so the plastic bonds well, and build up several thin layers of sandable primer to fill the sandpaper scratches in the bondo before a final sanding.