I cleaned the surface rust off the 531 frame and found a residual bubbling effect about three inches long and one inch wide on the underside of the down tube just before the b.b. juncture. This appears in a few other areas but very much smaller. What have I got here, a serious problem? I am trying to decide if it is worth sandblasting and repainting. BTW, the frame is a Raleigh Team Pro 531 DB, #SB 6130.

Thanks for your consideration.

bubbling? Pitting? depends on the depth of the damage and where it is, but I would be inclined not to put any money in a frame with substantial pitting from rust

are you talking about the bike in this thread:
https://www.bikeforums.net/showthread...tting-question
if so, why a new thread?

Is there access for water to get in the tube? Looks like it could be rusting from the inside out.