Originally Posted by
Andrew R Stewart
It will be interesting to find out if this stay was blind to the shell but still vented at the usual spot, a half inch ahead of it's drop out end. If so then a build up of water inside over many miles of wet conditions, or just the right outside storage, could produce a significant amount of water that wouldn't drip or evaporate out easily. I suspect that the stay will be nearly eaten away along it's bottom and up the sides a bit. If this frame were to be repaired I strongly suggest replacing the other stay and checking out the top tube's condition. Andy (who has drained 1/4 cup of water out of a frame a few times).
This frame needs a THOROUGH inspection before proceeding! If the chainstay is THIS bad, I'd expect lots more issues elsewhere. Inside bottom of the seat tube at the BB would be the next place to look. Even then, probably worth finding another bike or frame that's a better candidate for your time and effort. Unless it's pristine everywhere else, it's not worth taking a torch to. Even if it were, no framebuilder would accept a job like this. Repair work exceeds value of frame and liability. At least that's my take on the situation.