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.