Originally Posted by exas
inside, so much that some lines of rust can be seen under the paint
How long is a piece of string? It is like me saying - I have a spot of rust on my bike frame - is is going to fail? No it is just surface rust on my bike, nothing major for me.
Why don't you just remove the paint - check to see if it is just surface rust or runs a little deeper. If you don't know what you are looking for - then a rust hole is a bad thing to start with - major rust at joins is also bad I would assume. Take it to a frame builder if you are concerned.
If it is not too bad, search other threads for rust treatment.