I'll have to be a dissenter from most of the above. Here's a little story from a few years ago,
with pictures at
https://www.flickr.com/photos/131757...7627154051334/
"When cleaning up a Motobécane Grand Jubilé frame bought on eBay, I found a large wad of paper towel in the seat tube, below where the seat post was installed -- installed by previous owner (and eBay seller) who had probably believed this would preserve the bottom bracket (which rusted anyway, according to his eBay spiel). I had previously noticed a bubble on the seat tube, in one of the accent stripes, and had not thought much about it. I pushed out the wad of stuff using a dowel. There was a bunch of rust at the location of the wad, and I brushed it out as well as I could with a brass wire brush.
"I picked at the larger paint bubble on the exterior, and found that when the paint was scratched off (using only fingernail), there was a hole through the seat tube. A similar smaller bubble was probed with a pointed stick, and there was another hole there.
"Seller was unabashed, and unhelpful, even condescending, secure in the knowledge that the auction had taken place over 45 days from my finding the problem, so eBay was going to be of absolutely no help whatsoever.
"At my favorite local bike shop (Chelsea Bicycles on 26th St btw 6th & 7th Aves), which specializes in frame building and repairs, Rafael Vasquez took a piece of 28.6 downtube and slit about 3 mm out of one side. He then cleaned out the seat tube (not enlarging the holes much) wrapped the reinforcing tube around an undersized dowel, tied with thin wire to compress it, inserted it with epoxy into the seat tube, cutting each tie wire as it entered the seatpost hole. When cured, he reamed the interior of the reinforced area to 25.4 mm, and profiled the top of the reinforcement to match the top of the seat tube exactly (didn't even remove the paint on the lip).
"The fix added only about 35 g to the weight of the frame."
And the shim only took 1.2 mm off the seatpost size it accepts. You can see the finished bike here:
https://www.flickr.com/photos/131757...57627318121517
and a BF post about it here
http://www.bikeforums.net/classic-vi...nd-jubile.html
As always, YMMV.