Originally Posted by
taff78
Thanks for the reply. Yes it was my winter bike for a while and did not receive the care it should have. I think its just superficial rust. There is no sign of rust internally when looking in from the bottom bracket.
If I was doing the refinishing on this frame and it was mine, after sandblasting the area on the chain stays to get rid of the rust, I would fill the pits with silver. Those look like pretty deep pits. I wouldn't be worried that even rust holes that went all the way through would eventually break the chain stays if they stayed the same. What I would be worried about is that they are stress risers where cracks can start and continue when it is being ridden again.
The chain stay bridge is deeply rusted but the only reason to fill those pits (probably with body putty) is so it doesn't have a pitted look after repainting. There is too much steel in the bridge to be a structural problem.