when is some rust too much rust...
I bought a frame off eBay and it turns out that the seat tube shows some signs of rust: at the rear near the bottom bracket (nothing alarming) but also inside the seat tube (and this alarms me more)...
When I picked up the frame from the box I heard some loose stuff in the seat tube and quite a bit of rust/chunks of rusted metal fell off! This worried me but I took it to a trusted bike shop who told me that it was "not too bad". They knocked on the tube and it sounded ok to them, which reassured me for a bit, but later I did again scratch the inside of the tube with a metal hanger, and more "dust" and small chunks fell off.
So, I know nothing about rust: how it propagates, if it can be neutralised, stopped, etc. Basically the two scenarios I am looking at are: 1) repaint if the rust inside the tube can be treated 2) replacing the seat tube if the current one is compromised (no idea how much this will cost, I am expecting answers from the framebuilder who built the frame and a shop who does this kind of work... - yes, the frame is probably worth it)
On the one hand the rust would be at the most critical point of the frame, where most of the forces are applied, near the bottom bracket. On the other hand the seat tube is tapered, with the tube near the bottom bracket thicker, and giving me hope that there is plenty of metal untouched by the rust which can be stopped and treated.
It's really hard to take photos to show the condition of the tube, I have tried and photos look either way better or way worse than in person. Not to mention that I can't take photos from the inside of the frame.
My question... In the meantime, or short of sending the frame to the framebuilder for his diagnostic (he is super busy in the next months with all the orders for the next season), is there anything I can do to determine if the rust inside the tube is so bad that it compromises the frame?