I just bought a bike with a dent in the top tube (picrel). I bought the bike primarily for the groupset which is worth more than I paid for the entire bike, but the frame is lovey in its own right, a 1989 fillet brazed Columbus tubed race tier MTB, so if it's salvageable then I would love to keep it. I've seen non dented examples of this frame sell for upwards of £300-£400.
What gives me hope is I've seen a couple of videos where much worse dents have been repaired using frame blocks and bronze filler, so I'm optimistic that it might be OK.
https://www.youtube.com/watch?v=x4D_lLqj37cAny anons here had any experience with having dents repaired?
