Before:
Burley tandem circa 1990? Nice-ish bike but they didn't drill drain holes in the BB shell. Water got in, maybe while the bike was being transported on a car rack. Had no place to go, so it pooled at the bottom and gradually ate itself a drain hole.
The bike was slightly too nice to throw away, and I need a hobby, so I fixed it. I re-mitered all 3 tubes and replaced the BB shell with a larger one. The too-small BB shell (not enough eccentricity), that used grub-screws <ick!> to set the eccentric, was the one thing I kinda hated about this era of Burley. Going to a larger shell, and offsetting the miter upward, meant most of the rusted seattube was mitered away.
I also made a sleeve to go over the seattube, tapered so it's thick at the bottom and feathering to nothing at the top, so the repair isn't visible after painting. And I made the brass fillets kinda large to make extra-sure all the rusty part is well strengthened.
After:
The complete story including pics of the assembled bike is
here (Flickr)
Mark B