Today I was doing some touch-up (with clear nail polish) on my 1987 Centurion Ironman Expert frame (Tange #1 butted cromoly tubing), and noticed a crack in the paint along the lugged joint between the seat tube and bottom bracket. The crack is on the back of the seat tube, toward the left (non-drive-side). It's a slight crack in the paint, and I don't know if it heralds any danger of a crack in the brass brazing material that's directly underneath the paint at that point.
I bought the bike off of eBay, but it's been in good shape and so far as I know has never been crashed. I did slightly spread the frame's rear triangle (using the cold setting method recommended by Sheldon Brown) but only went from 126mm to 128mm spacing. I think of this only because Sheldon's method requires the lever arm to be braced against the seat tube in order to apply force. But it's not like I stomped on the frame or anything; I was quite careful.
I'm guessing this isn't something to be very worried about (though I'll be sure to watch it) but would like to hear feedback. I know it's possible to sand away the paint just at that point and look at the brazed joint, which I may do but would rather not if it's unnecessary.
Anyway, here are three successive pictures zooming in on the same shot.